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Dosimetric evaluation of parallel opposed spatially fractionated radiation therapy (GRID) for deep-seated bulky tumors
Abstract
Application of a single fraction of parallel opposed GRID beams as a means of increasing the efficiency of radiation delivery to deep-seated tumors has been investigated. This evaluation was performed by measurement of dosimetric characteristics of the GRID radiation field in parallel opposed and single beam geometry. The limitations of the parallel opposed technique in terms of field size and tumor thickness have been evaluated for the conditions of acceptable spatial modulation. The results of this investigation have demonstrated an increase in therapeutic advantage for the parallel opposed technique over the single beam method when treating a deep seated tumor.
... Validation of the dose calculation accuracy of the treatment planning system (TPS) in the build-up region for small fields was performed by film dosimetry. The technique and the materials used for the validation were identical to the materials and methods described in the literature [13]. Briefly, Kodak X-Omat V radiographic film (Eastman Kodak, Rochester, NY) was irradiated in a solid water (Radiation Measurements Inc. Middleton, WI) phantom. ...
... difference between the measured and the calculated doses is within ±3%, which is comparable to the accuracy achievable with film dosimetry [13]. Notably, Spezi et al. [11] reported that Pinnacle CC algorithm reproduces ion-chamber measured doses in the build-up region to within 2% at a depth beyond 0.5 cm, which is consistent with our findings since the CFC water-equivalent thickness is 1.1 cm. ...
... In addition, it is also apparent from the data that the maximum of the D 1% saturates around $60%, in other words, with an increasing number of beams the maximum value of the DI stops changing substantially. Notably, the skin doses as a fraction of the prescription doses will be higher in the 6 MV plans as can be inferred form Fig. 2. Skin doses as high as 70% (or even higher) of the prescription dose observed in individual cases imply that especially in high-dose-perfraction stereotactic radiosurgery (SRS), stereotactic radiotherapy (SRT), or GRID [13,34] treatments the effect of CFC on skin doses needs to be carefully evaluated and mixed energy plans need to be considered. The latter finding supports the recently published hypothesis [27] where it was implied that skin doses greater than 50% of the prescription dose result in acute skin reactions in stereotactic radiation therapy. ...
To quantify the skin doses resulting from the use of carbon fiber couches (CFCs) for patient support.
BrainLab's CFC was evaluated for five prostate patients and five lung patients. For each patient PTV, organs at risk (OARs), and a 0.3cm thick skin contour on the patient's posterior surface were outlined. Two sets of IMRT plans, each consisting of 4, 5, 7, and 9 beams, were generated per patient. The sets were identical with the exception that in the first set only 6MV energy was used, while in the second set (mixed energy) the photon energy of the beams traversing the CFC was 18MV. The plans for each patient were normalized to deliver the same dose to 95% of the PTV. The CFC skin dose was evaluated by the maximum dose received by 1% (D(1%)) of the skin volume. Paired one-tailed t-tests were used to establish the statistical significance.
The mixed energy plans resulted in D(1%) increase from 18% to more than 23% as the number of beams in the plan was decreased from 9 to 4.
Skin doses as high as approximately 70% of the prescription dose were found even in 9-beam mixed energy plans. Therefore mixed energy plans may be more beneficial for patients treated with higher fractional doses.
... Ha et al. (15) studied the feasibility of grid therapy delivery using an MLC. Meigooni et al. (16,17) measured the dose distributions of 6-MV and 18-MV photon beams modulated by a hexagonal grid using films, thermoluminescent dosimeters, and an ion chamber in water. Zhang et al. (18) derived two-dimensional (2D) dose distributions of 6-MV grid therapy with the Monte Carlo (MC) simulations, extended dose range (EDR2) films in solid water phantom, and an ion chamber in water scanning system. ...
... The structure of the grid has already been described in detail (16)(17)(18). In brief, the grid used in this study (High Dose Radiation Grid, Radiation Products Design, Albertville, MN) consisted of a 7.5-cmthick Cerrobend block perforated by a hexagonal pattern (Fig. 1) of circular divergent holes and designed to be mounted in the standard linear accelerator accessory mount 65.4 cm from the source (Varian 21 EX, Varian Oncology System, Palo Alto, CA). ...
... All recent studies (15)(16)(17)(18) involving grid therapy have investigated a hexagonal grid collimator with aperture diameters of approximately 1 cm projected in the plane of the isocenter. Although a grid collimator with an aperture of this size has been used in clinical treatments and showed encouraging outcomes, there is little evidence to show that this design represents optimal grid collimation for megavoltage applications. ...
To evaluate the conventionally fractionated and hypofractionated grid therapy in debulking cervical cancers using the linear quadratic (LQ) model.
A Monte Carlo technique was used to calculate the dose distribution of a commercially available grid in a 6-MV photon beam. The LQ model was used to evaluate the therapeutic outcome of both the conventionally fractionated (2 Gy/fraction) and hypofractionated (15 Gy/fraction) grid therapy regimens to debulk cervical cancers with different LQ parameters. The equivalent open-field dose (EOD) to the cancer cells and therapeutic ratio (TR) were defined by comparing grid therapy with the open debulking field. The clinical outcomes from 114 patients were used to verify our theoretical model.
The cervical cancer and normal tissue cell survival statistics for grid therapy in two regimens were calculated. The EODs and TRs were derived. The EOD was only a fraction of the prescribed dose. The TR was dependent on the prescribed dose and the LQ parameters of both the tumor and normal tissue cells. The grid therapy favors the acutely responding tumors inside radiosensitive normal tissues. Theoretical model predictions were consistent with the clinical outcomes.
Grid therapy provided a pronounced therapeutic advantage in both the hypofractionated and conventionally fractionated regimens compared with that seen with single fraction, open debulking field regimens, but the true therapeutic advantage exists only in the hypofractionated grid therapy. The clinical outcomes and our study indicated that a course of open-field radiotherapy is necessary to control tumor growth fully after a grid therapy.
... An interesting application in the field of MV radiotherapy involves the creation of multi-collimator blocks for spatially fractionated radiotherapy (SFRT). SFRT is a treatment method that involves administering high doses of radiation in a single fraction to a target volume in a heterogeneous manner using individual thin beams [31]. This approach utilizes accessories like grid-shaped blocks made with high-density materials, offering advantages for specific pathologies not achievable with modern techniques [31][32][33]. ...
... SFRT is a treatment method that involves administering high doses of radiation in a single fraction to a target volume in a heterogeneous manner using individual thin beams [31]. This approach utilizes accessories like grid-shaped blocks made with high-density materials, offering advantages for specific pathologies not achievable with modern techniques [31][32][33]. These grid collimators were traditionally constructed using CNC machining techniques outside hospital environments or purchased commercially [34] at a high cost as generic grids without customization by pathology or patient. ...
This work evaluates the radiation shielding capabilities of the PLA-W composite for MV energy photons emitted by a linear accelerator and the feasibility of manufacturing a clinically-used collimator grid in spatially fractionated radiotherapy (SFRT) using the material extrusion (MEX) 3D printing technique. The PLA-W filament used has a W concentration of 93% w/w and a green density of 7.51 g/cm3, characteristics that make it suitable for this purpose. Relevant parameters such as the density and homogeneity distribution of W in the manufactured samples determine the mass attenuation coefficient, directly affecting the radiation shielding capacities, so different printing parameters were evaluated, such as layer height, deposition speed, nozzle temperature, and infill, to improve the protection performance of the samples. Additionally, physical and mechanical tests were conducted to ensure structural stability and spatial variability over time, which are critical to ensure precise spatial modulation of radiation. Finally, a complete collimator grid measuring 9.3 × 9.3 × 7.1 cm3 (consisting of 39 conical collimators with a diameter of 0.92 cm and center-to-center spacing of 1.42 cm) was manufactured and experimentally evaluated on a clinical linear accelerator to measure the radiation shielding and dosimetric parameters such as mass attenuation coefficient, half-value layer (HVL), dosimetric collimator field size, and inter-collimator transmission using radiochromic films and 2D diode array detectors, obtaining values of 0.04692 cm2/g, 2.138 cm, 1.40 cm, and 15.6%, respectively, for the parameters in the study. This shows the viability of constructing a clinically-used collimator grid through 3D printing.
... However, the depth of the tumour center may be variable, and the treating physician may prescribe the dose to a depth (d) other than dmax. In addition, if two opposed GRID fields are used [18] then the combined PDD curves would be entirely different. If this is the case, the GRID field output factor must be measured at the depth d. ...
... However, the depth of the tumour center may be variable, and the treating physician may prescribe the dose to a depth (d) other than dmax. In addition, if two opposed GRID fields are used [18] then the combined PDD curves would be entirely different. If this is the Cancers 2022, 14, 1037 6 of 17 case, the GRID field output factor must be measured at the depth d. ...
Simple Summary
Dose prescription for the inhomogeneous dosing in spatially fractionated radiation therapy (SFRT) is challenging, and further hampered by the inability of several planning systems to incorporate complex SFRT dose patterns. We developed dosing reference tables for an inventory of tumour scenarios and tested their accuracy with water phantom measurements of GRID therapy, delivered by a standard commercial GRID collimator. We find that dose heterogeneity parameters and EUD modeling are consistent across tumour sizes, configurations, and treatment depths. These results suggest that the developed reference tables can be used as a practical clinical resource for clinical decision-making on GRID therapy and to facilitate heterogeneity dose estimates in clinical patients when this commercially available GRID device is used.
Abstract
Computations of heterogeneity dose parameters in GRID therapy remain challenging in many treatment planning systems (TPS). To address this difficulty, we developed reference dose tables for a standard GRID collimator and validate their accuracy. The .decimal Inc. GRID collimator was implemented within the Eclipse TPS. The accuracy of the dose calculation was confirmed in the commissioning process. Representative sets of simulated ellipsoidal tumours ranging from 6–20 cm in diameter at a 3-cm depth; 16-cm ellipsoidal tumours at 3, 6, and 10 cm in depth were studied. All were treated with 6MV photons to a 20 Gy prescription dose at the tumour center. From these, the GRID therapy dosimetric parameters (previously recommended by the Radiosurgery Society white paper) were derived. Differences in D5 through D95 and EUD between different tumour sizes at the same depth were within 5% of the prescription dose. PVDR from profile measurements at the tumour center differed from D10/D90, but D10/D90 variations for the same tumour depths were within 11%. Three approximation equations were developed for calculating EUDs of different prescription doses for three radiosensitivity levels for 3-cm deep tumours. Dosimetric parameters were consistent and predictable across tumour sizes and depths. Our study results support the use of the developed tables as a reference tool for GRID therapy.
... Currently, MV GRID therapy treatments are delivered using a high attenuation GRID-block with divergent holes, with step and shoot multileaf collimator (MLC) control points, and/or Tomotherapy machines. [4][5][6][7][8][9][10][11][12][13][14][15] Although, the treatment planning studies for GRID therapy using tomotherapy and step and shoot MLCs are evolving, these techniques require longer treatment times due to beam modulation and need patient-specific quality assurance (QA). Moreover, noninterdigitating MLCs potentially may not allow an efficient implementation of this method. ...
... Therefore, the plan needed additional treatment planning and optimization time as well as patient-specific QA due to MLC modulation. A major difference of our study from the previous two-dimensional-GRID therapy approach, 1-3,16,22 tomotherapy or MLC-based studies [4][5][6][7][8][9][10][11][12][13][14][15] was that our treatment planning approach uses an MLCbased, 3D-conformal forward planning technique with no beam modulation. Therefore, this MLC cross-firing procedure preserves the characteristics of 3D-conformal radiation therapy and provides all dosimetry information without the need for patient-specific QA. ...
Purpose:
Treating deep-seated bulky tumors with traditional single-field Cerrobend GRID-blocks has many limitations such as suboptimal target coverage and excessive skin toxicity. Heavy traditional GRID-blocks are a concern for patient safety at various gantry-angles and dosimetric detail is not always available without a GRID template in user's treatment planning system. Herein, we propose a simple, yet clinically useful multileaf collimator (MLC)-based three-dimensional (3D)-crossfire technique to provide sufficient target coverage, reduce skin dose, and potentially escalate tumor dose to deep-seated bulky tumors.
Materials/methods:
Thirteen patients (multiple sites) who underwent conventional single-field cerrobend GRID-block therapy (maximum, 15 Gy in 1 fraction) were re-planned using an MLC-based 3D-crossfire method. Gross tumor volume (GTV) was used to generate a lattice pattern of 10 mm diameter and 20 mm center-to-center mimicking conventional GRID-block using an in-house MATLAB program. For the same prescription, MLC-based 3D-crossfire grid plans were generated using 6-gantry positions (clockwise) at 60° spacing (210°, 270°, 330°, 30°, 90°, 150°, therefore, each gantry angle associated with a complement angle at 180° apart) with differentially-weighted 6 or 18 MV beams in Eclipse. For each gantry, standard Millenium120 (Varian) 5 mm MLC leaves were fit to the grid-pattern with 90° collimator rotation, so that the tunneling dose distribution was achieved. Acuros-based dose was calculated for heterogeneity corrections. Dosimetric parameters evaluated include: mean GTV dose, GTV dose heterogeneities (peak-to-valley dose ratio, PVDR), skin dose and dose to other adjacent critical structures. Additionally, planning time and delivery efficiency was recorded. With 3D-MLC, dose escalation up to 23 Gy was simulated for all patient's plans.
Results:
All 3D-MLC crossfire GRID plans exhibited excellent target coverage with mean GTV dose of 13.4 ± 0.5 Gy (range: 12.43-14.24 Gy) and mean PVDR of 2.0 ± 0.3 (range: 1.7-2.4). Maximal and dose to 5 cc of skin were 9.7 ± 2.7 Gy (range: 5.4-14.0 Gy) and 6.3 ± 1.8 Gy (range: 4.1-11.1 Gy), on average respectively. Three-dimensional-MLC treatment planning time was about an hour or less. Compared to traditional GRID-block, average beam on time was 20% less, while providing similar overall treatment time. With 3D-MLC plans, tumor dose can be escalated up to 23 Gy while respecting skin dose tolerances.
Conclusion:
The simple MLC-based 3D-crossfire GRID-therapy technique resulted in enhanced target coverage for de-bulking deep-seated bulky tumors, reduced skin toxicity and spare adjacent critical structures. This simple MLC-based approach can be easily adopted by any radiotherapy center. It provides detailed dosimetry and a safe and effective treatment by eliminating the heavy physical GRID-block and could potentially provide same day treatment. Prospective clinical trial with higher tumor-dose to bulky deep-seated tumors is anticipated.
... Cross-firing of beam grids, containing either ion or photon beam elements, over a target volume has previously been suggested. [22][23][24][25] In previous studies of proton beam grid therapy, the aim has normally been to create a uniform target dose. 15,16,23 On the contrary, when photon beam grids have been considered for cross-firing, the aim has often been to produce a highly nonuniform target dose, reminiscent of what can be created in brachytherapy. ...
... 15,16,23 On the contrary, when photon beam grids have been considered for cross-firing, the aim has often been to produce a highly nonuniform target dose, reminiscent of what can be created in brachytherapy. 22,24,25 It is evident that it is possible to create nonuniform target doses also with proton beam grid irradiations. However, existing tumor control probability models indicate that an improved therapeutic effect can be obtained if the minimum target dose is sufficiently elevated. ...
In this work, we studied the possibility of merging proton therapy with grid therapy. We hypothesized that patients with larger targets containing solid tumor growth could benefit from being treated with this method, proton grid therapy. We performed treatment planning for 2 patients with abdominal cancer with the suggested proton grid therapy technique. The proton beam arrays were cross-fired over the target volume. Circular or rectangular beam element shapes (building up the beam grids) were evaluated in the planning. An optimization was performed to calculate the fluence from each beam grid element. The optimization objectives were set to create a homogeneous dose inside the target volume with the constraint of maintaining the grid structure of the dose distribution in the surrounding tissue. The proton beam elements constituting the grid remained narrow and parallel down to large depths in the tissue. The calculation results showed that it is possible to produce target doses ranging between 100% and 130% of the prescribed dose by cross-firing beam grids, incident from 4 directions. A sensitivity test showed that a small rotation or translation of one of the used grids, due to setup errors, had only a limited influence on the dose distribution produced in the target, if 4 beam arrays were used for the irradiation. Proton grid therapy is technically feasible at proton therapy centers equipped with spot scanning systems using existing tools. By cross-firing the proton beam grids, a low tissue dose in between the paths of the elemental beams can be maintained down to the vicinity of a deep-seated target. With proton grid therapy, it is possible to produce a dose distribution inside the target volume of similar uniformity as can be created with current clinical methods.
... LINAC-GRID usually uses one beam angle or two beams in parallel opposed fashion. (7) The details of LINAC-GRID plans and their beam profiles were studied in our previous publications. (3,6) B. HT-GRID technique treatment plan ...
... The literature provides data regarding the use of parallel opposed GRID therapy to improve upon the dose distribution of the GRID GTV when treating deep-seated tumors. Meigooni et al. (7) reported on GRID therapy using parallel opposed beams. It was shown that parallel opposed GRID is a viable option for treatment of deep-seated tumors as the spatially fractionated dose distribution is preserved. ...
Spatially fractionated radiotherapy (GRID) was designed to treat large tumors while sparing skin, and it is usually delivered with a linear accelerator using a commercially available block or multileaf collimator (LINAC‐GRID). For deep‐seated (skin to tumor distance ) tumors, it is always a challenge to achieve adequate tumor dose coverage. A novel method to perform GRID treatment using helical tomotherapy (HT‐GRID) was developed at our institution. Our approach allows treating patients by generating a patient‐specific virtual GRID block (software‐generated) and using IMRT technique to optimize the treatment plan. Here, we report our initial clinical experience using HT‐GRID, and dosimetric comparison results between HT‐GRID and LINAC‐GRID. This study evaluates 10 previously treated patients who had deep‐seated bulky tumors with complex geometries. Five of these patients were treated with HT‐GRID and replanned with LINAC‐GRID for comparison. Similarly, five other patients were treated with LINAC‐GRID and replanned with HT‐GRID for comparison. The prescription was set such that the maximum dose to the GTV is 20 Gy in a single fraction. Dosimetric parameters compared included: mean GTV dose ( ), GTV dose inhomogeneity (valley‐to‐peak dose ratio (VPR)), normal tissue doses ( ), and other organs‐at‐risk (OARs) doses. In addition, equivalent uniform doses (EUD) for both GTV and normal tissue were evaluated. In summary, HT‐GRID technique is patient‐specific, and allows adjustment of the GRID pattern to match different tumor sizes and shapes when they are deep‐seated and cannot be adequately treated with LINAC‐GRID. HT‐GRID delivers a higher , EUD, and VPR compared to LINAC‐GRID. HT‐GRID delivers a higher and lower EUD for normal tissue compared to LINAC‐GRID. HT‐GRID plans also have more options for tumors with complex anatomical relationships between the GTV and the avoidance OARs (abutment or close proximity).
PACS numbers: 87.55.D, 87.55.de, 87.55.ne, 87.55.tg
... Spatially fractionated radiation therapy using GRID therapy is utilized to treat large tumors by irradiating only the volume of tumor exposed through isolated small openings (Meigooni et al., 2007;Mohiuddin et al., 1990Mohiuddin et al., , 1999. The technique has shown high efficacy for bulky tumors receiving doses of 10-20 Gy in a single fraction (Mohiuddin et al., 1990). ...
... Historically, dense GRID collimators used for the radiation delivery are attached to the gantry (Huhn et al., 2006;Meigooni et al., 2007Meigooni et al., , 2002; Mohiuddin et al., 1990Mohiuddin et al., , 1999Zwicker et al., 2004). Although the results are promising, only a small number of institutions practice GRID therapy. ...
In this paper, we present an alternative to the originally proposed technique for the delivery of spatially fractionated radiation therapy (GRID) using multi-leaf collimator (MLC) shaped fields. We employ the MLC to deliver various pattern GRID treatments to large solid tumors and dosimetrically characterize the GRID fields.
The GRID fields were created with different open to blocked area ratios and with variable separation between the openings using a MLC. GRID designs were introduced into the Pinnacle(3) treatment planning system, and the dose was calculated in a water phantom. Ionization chamber and film measurements using both Kodak EDR2 and Gafchromic EBT film were performed in a SolidWater phantom to determine the relative output of each GRID design as well as its spatial dosimetric characteristics.
Agreement within 5.0% was observed between the Pinnacle(3) predicted dose distributions and the measurements for the majority of experiments performed. A higher magnitude of discrepancy (15%) was observed using a high photon beam energy (18MV) and small GRID opening. Skin dose at the GRID openings was higher than the corresponding open field by a factor as high as three for both photon energies and was found to be independent of the open-to-blocked area ratio.
In summary, we reaffirm that the MLC can be used to deliver spatially fractionated GRID therapy and show that various GRID patterns may be generated. The Pinnacle(3) TPS can accurately calculate the dose of the different GRID patterns in our study to within 5% for the majority of the cases based on film and ion chamber measurements. Disadvantages of MLC-based GRID therapy are longer treatment times and higher surface doses.
... Pencil-beam scanning (PBS) proton therapy has been used to improve dose distributions in proof-of-concept models, dosimetry studies [8,9], and in the clinical setting with a sample size of 10 patients [10]. These methods used single-field PBS with spot pattern mimicking the brass collimation block, which historically has been the most commonly used modality in the clinical setting [4,11]. This method results in a significant portion of high dose delivered superficially, thereby limiting its utility for deeper tumors. ...
Purpose
To compare spatially fractionated radiation therapy (GRID) treatment planning techniques using proton pencil-beam-scanning (PBS) and photon therapy.
Materials and Methods
PBS and volumetric modulated arc therapy (VMAT) GRID plans were retrospectively generated for 5 patients with bulky tumors. GRID targets were arranged along the long axis of the gross tumor, spaced 2 and 3 cm apart, and treated with a prescription of 18 Gy. PBS plans used 2- to 3-beam multiple-field optimization with robustness evaluation. Dosimetric parameters including peak-to-edge ratio (PEDR), ratio of dose to 90% of the valley to dose to 10% of the peak VPDR(D90/D10), and volume of normal tissue receiving at least 5 Gy (V5) and 10 Gy (V10) were calculated. The peak-to-valley dose ratio (PVDR), VPDR(D90/D10), and organ-at-risk doses were prospectively assessed in 2 patients undergoing PBS-GRID with pretreatment quality assurance computed tomography (QACT) scans.
Results
PBS and VMAT GRID plans were generated for 5 patients with bulky tumors. Gross tumor volume values ranged from 826 to 1468 cm3. Peak-to-edge ratio for PBS was higher than for VMAT for both spacing scenarios (2-cm spacing, P = .02; 3-cm spacing, P = .01). VPDR(D90/D10) for PBS was higher than for VMAT (2-cm spacing, P = .004; 3-cm spacing, P = .002). Normal tissue V5 was lower for PBS than for VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Normal tissue mean dose was lower with PBS than with VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Two patients treated using PBS GRID and assessed with pretreatment QACT scans demonstrated robust PVDR, VPDR(D90/D10), and organs-at-risk doses.
Conclusions
The PEDR was significantly higher for PBS than VMAT plans, indicating lower target edge dose. Normal tissue mean dose was significantly lower with PBS than VMAT. PBS GRID may result in lower normal tissue dose compared with VMAT plans, allowing for further dose escalation in patients with bulky disease.
... However, the depth of the tumor center may be variable, and the treating physician may decide to prescribe the dose to a depth (d) other than dmax. In addition, if two opposed GRID fields are used (39), then the PDD curves would be entirely different. If this is the case, then the physicist must measure or calculate the GRID field output factor at d. Again, the calculated output factor needs to be experimentally verified. ...
The limits of radiation tolerance, which often deter the use of large doses, have been a major challenge to the treatment of bulky primary and metastatic cancers. A novel technique using spatial modulation of megavoltage therapy beams, commonly referred to as spatially fractionated radiation therapy (SFRT) (e.g., GRID radiation therapy), which purposefully maintains a high degree of dose heterogeneity across the treated tumor volume, has shown promise in clinical studies as a method to improve treatment response of advanced, bulky tumors. Compared to conventional uniform-dose radiotherapy, the complexities of megavoltage GRID therapy include its highly heterogeneous dose distribution, very high prescription doses, and the overall lack of experience among physicists and clinicians. Since only a few centers have used GRID radiation therapy in the clinic, wide and effective use of this technique has been hindered. To date, the mechanisms underlying the observed high tumor response and low toxicity are still not well understood. To advance SFRT technology and planning, the Physics Working Group of the Radiosurgery Society (RSS) GRID/Lattice, Microbeam and Flash Radiotherapy Working Groups, was established after an RSS-NCI Workshop. One of the goals of the Physics Working Group was to develop consensus recommendations to standardize dose prescription, treatment planning approach, response modeling and dose reporting in GRID therapy. The objective of this report is to present the results of the Physics Working Group's consensus that includes recommendations on GRID therapy as an SFRT technology, field dosimetric properties, techniques for generating GRID fields, the GRID therapy planning methods, documentation metrics and clinical practice recommendations. Such understanding is essential for clinical patient care, effective comparisons of outcome results, and for the design of rigorous clinical trials in the area of SFRT. The results of well-conducted GRID radiation therapy studies have the potential to advance the clinical management of bulky and advanced tumors by providing improved treatment response, and to further develop our current radiobiology models and parameters of radiation therapy design.
... Some investigators have tried to mitigate this dosimetric problem with the use of parallel opposed fields. 6 Another solution, which maintains the unique geometry of high-dose "islands" within tumors inherent with GRID and takes advantage of high-energy x-ray attenuation features, is to paint three-dimensional target structures throughout the tumor which mimic conventional two-dimensional GRID blocks and then use conformal or intensity-modulated planning with the goal to deliver high doses to these areas (instead of the entire tumor). This approach has previously been studied using tomotherapy. ...
Purpose:
The purpose of this work was to compare the dosimetry and delivery times of 3D-conformal (3DCRT)-, volumetric modulated arc therapy (VMAT)-, and tomotherapy-based approaches for spatially fractionated radiation therapy for deep tumor targets.
Methods:
Two virtual GRID phantoms were created consisting of 7 "target" cylinders (1-cm diameter) aligned longitudinally along the tumor in a honey-comb pattern, mimicking a conventional GRID block, with 2-cm center-to-center spacing (GRID2 cm ) and 3-cm center-to-center spacing (GRID3 cm ), all contained within a larger cylinder (8 and 10 cm in diameter for the GRID2 cm and GRID3 cm , respectively). In a single patient, a GRID3 cm structure was created within the gross tumor volume (GTV). Tomotherapy, VMAT (6 MV + 6 MV-flattening-filter-free) and multi-leaf collimator segment 3DCRT (6 MV) plans were created using commercially available software. Two tomotherapy plans were created with field widths (TOMO2.5 cm ) 2.5 cm and (TOMO5 cm ) 5 cm. Prescriptions for all plans were set to deliver a mean dose of 15 Gy to the GRID targets in one fraction. The mean dose to the GRID target and the heterogeneity of the dose distribution (peak-to-valley and peak-to-edge dose ratios) inside the GRID target were obtained. The volume of normal tissue receiving 7.5 Gy was determined.
Results:
The peak-to-valley ratios for GRID2 cm /GRID3 cm /Patient were 2.1/2.3/2.8, 1.7/1.5/2.8, 1.7/1.9/2.4, and 1.8/2.0/2.8 for the 3DCRT, VMAT, TOMO5 cm , and TOMO2.5 cm plans, respectively. The peak-to-edge ratios for GRID2 cm /GRID3 cm /Patient were 2.8/3.2/5.4, 2.1/1.8/5.4, 2.0/2.2/3.9, 2.1/2.7/5.2 and for the 3DCRT, VMAT, TOMO5 cm , and TOMO2.5 cm plans, respectively. The volume of normal tissue receiving 7.5 Gy was lowest in the TOMO2.5 cm plan (GRID2 cm /GRID3 cm /Patient = 54 cm3 /19 cm3 /10 cm3 ). The VMAT plans had the lowest delivery times (GRID2 cm /GRID3 cm /Patient = 17 min/8 min/9 min).
Conclusion:
Our results present, for the first time, preliminary evidence comparing IMRT-GRID approaches which result in high-dose "islands" within a target, mimicking what is achieved with a conventional GRID block but without high-dose "tail" regions outside of the target. These approaches differ modestly in their ability to achieve high peak-to-edge ratios and also differ in delivery times.
... Spatially fractionated radiotherapy or Grid therapy is a radiation therapy technique for the treatment of bulky and advanced tumours (Mohiuddin et al., 1990) While having therapeutic advantages for normal tissues, this treatment technique is usually used to deliver the required radiation dose with a linear accelerator by using a designed block collimator (Meigooni et al., 2007) A Grid field represents a group of pencil beams giving a high radiation dose (Zhang et al., 2006) Generally, treatments are performed with a single fraction of 10 Gy-20 Gy, prescribed to the depth of the maximum dose, followed by conventional radiotherapy (Zhang et al., 2006). The valley-to-peak dose ratio (VPDR) in the dose distribution of this technique is defined as the ratio of dose in the blocked and the open areas. ...
Objectives: Grid therapy is a radiation therapy technique for the treatment of bulky tumours. The high dose gradients and non-uniformity of dose distributions within the target lead to a challenge in the dosimetry of the Grid radiation fields. The aim of this study is to perform a precise EBT3 Gafchromic film dosimetry for Grid therapy fields using a commercially available flatbed colour scanner. Methods: In this project, samples of the EBT3 Gafchromic films are exposed to Grid radiation fields. The irradiated EBT3 films were read using a flatbed Microtek scanner. The responses of these films (i.e. films from the same batch) as functions of the absorbed dose values are calibrated by irradiation under a fixed standard technique (i.e. 10 × 10 cm² filed, 100 cm SSD, and depth of the maximum dose). These films are also read with the same scanner using the red, green, and blue channels. Four different approaches were used to evaluate film dosimetry for the Grid therapy applications: 1) single channel film dosimetry method (SCM), 2) dual channel film dosimetry method (DCM), 3) linearized dose-response curve method (LRCM), and 4) triple channel film dosimetry method (TCM). A dose of 20 Gy was delivered to the point along the central axis of the grid hole at the depth of maximum dose (dmax) for a 20 × 20 cm² Grid field size. Beam profiles and percentage depth dose distributions of the Grid radiation have been measured in water-equivalent phantom material, using EBT3 films. The accuracy of the relative and the absolute dosimetry of the films were examined by comparison of the TLD measured data with the Monte Carlo simulated values. Results: The results of these investigations show that for a gamma index criterion of 5%/3 mm, the agreements between the MC calculations dose profiles and the SCM, DCM, LRCM, and TCM film dosimetry approaches the passing rates of 91%, 92%, 95%, and 95%, respectively. A much closer agreement was observed for using a linearized dose-response curve and triple-channel methods. Conclusions: Selection of an appropriate methodology in Gafchromic film dosimetry may lead to an accurate dose-response in a high dose gradient radiation field such as Grid therapy.
... Megavoltage grid therapy itself has over six decades of history, spanning multiple tumor types and sites. [3][4][5][6][7][8][9][10][11] With either a curative or palliative intent, grid therapy is often delivered in a single fraction with 15-20 Gy at the depth of maximum dose (d max ). Skin is the organ at risk during grid therapy, and the clinical benefit of skin sparing is achieved by delivering a beam of a grid pattern. ...
Purpose:
Grid therapy has promising applications in the radiation treatment of large tumors. However, research and applications of grid therapy are limited by the accessibility of the specialized blocks that produce the grid of pencil-like radiation beams. In this study, a Cerrobend grid block was fabricated using the 3D printing technique.
Methods:
A grid block mold was designed with flared tubes which follow the divergence of the beam. The mold was 3D printed using a resin with the working temperature below 230 °C. The melted Cerrobend liquid at 120 °C was cast into the resin mold to yield a block with a thickness of 7.4 cm. At the isocenter plane, the grid had a hexagonal pattern, with each pencil beam diameter of 1.4 cm; the distance between the beam centers was 2.1 cm.
Results:
The dosimetric properties of the grid block were studied using small field dosimeters: a pinpoint ionization chamber and a stereotactic diode. For a 6 MV photon beam, its valley-to-peak ratio was 20% at d max and 30% at 10 cm depth; the output factor was 84.9% at d max and 65.1% at 10 cm depth.
Conclusions:
This study demonstrates that it is feasible to implement 3D printing technique in applying grid therapy in clinic.
... Megavoltage grid therapy itself has over six decades of history, spanning multiple tumor types and sites. [3][4][5][6][7][8][9][10][11] With either a curative or palliative intent, grid therapy is often delivered in a single fraction with 15-20 Gy at the depth of maximum dose (d max ). Skin is the organ at risk during grid therapy, and the clinical benefit of skin sparing is achieved by delivering a beam of a grid pattern. ...
Grid therapy has promising applications in the radiation treatment of bulky and large tumors. However, research and applications of grid therapy is limited by the accessibility of the specialized blocks that produce the grid of pencil-like radiation beams. In this study, a Cerrobend grid block was fabricated using a 3D printing technique.
A grid block mold was designed with divergent tubes following beam central rays. The mold was printed using a resin with the working temperature below 230 °C. The melted Cerrobend liquid at 120°oC was cast into the resin mold to yield a block with a thickness of 7.4 cm. The grid had a hexagonal pattern, with each pencil beam diameter of 1.4 cm at the iso-center plane; the distance between the beam centers was 2 cm. The dosimetric properties of the grid block were studied using radiographic film and small field dosimeters.
the grid block was fabricated to be mounted at the third accessory mount of a Siemens Oncor linear accelerator. Fabricating a grid block using 3D printing is similar to making cutouts for traditional radiotherapy photon blocks, with the difference being that the mold was created by a 3D printer rather than foam. In this study, the valley-to-peak ratio for a 6MV photon grid beam was 20% at dmax, and 30% at 10 cm depth, respectively.
We have demonstrated a novel process for implementing grid radiotherapy using 3D printing techniques. Compared to existing approaches, our technique combines reduced cost, accessibility, and flexibility in customization with efficient delivery. This lays the groundwork for future studies to improve our understanding of the efficacy of grid therapy and apply it to improve cancer treatment.
... It is obvious from the data that rather large skin doses may result in either case. Skin doses as high as $80% of the prescription dose observed in individual cases imply that especially in high-dose-per-fraction stereotactic radiosurgery (SRS), stereotactic radiotherapy (SRT), or GRID 43,44 treatment the effect of CFC on skin doses need to be carefully evaluated and mixed energy plans considered in VMAT treatments. Furthermore, a counterintuitive fact is evident from the data. ...
The purpose of this study is to evaluate the dosimetric effect of carbon fiber couches (CFCs) on delivered skin dose as well as to explore potential venues for its minimization for volumetric modulated arc (VMAT) treatments.
A carbon fiber couch (BrainLab) was incorporated in Pinnacle treatment planning system (TPS) by autocontouring. A retrospective investigation on five lung and five prostate patient plans was performed. Targets and organs at risk (OARs), together with a 0.3 cm thick skin contour interfacing the CFC, were outlined in each plan. For each patient, two VMAT plans were generated: a single arc with 6 MV photon energy and two or three arcs with 18 MV photon energy for the posterior arc(s) and 6 MV energy for the anterior arc (mixed energy plans). Both plans for each patient case were normalized such that 95% of the PTV was covered by the same prescription dose, ranging from 7600 to 7800 cGy. For each patient, the prescription doses were escalated to the maximum allowed by the OAR constraints. CFC bolus effects on skin doses were tallied by the highest dose to 1% of skin volume.
With the utilization of higher energy photons for the posterior arcs, the statistically significant differences in skin dose between the two plans were as high as 34% of the prescribed dose, where surface doses changed on average from 3800 to 2940 cGy for 6 MV and mixed energy plans, respectively. In addition, skin doses in excess of 68% and 80% of the prescription doses for mixed and 6 MV energy plans, respectively, were observed in individual cases.
The presented findings indicate that mixed energy VMAT plans would result in a substantial skin sparing of more than approximately 34% compared to VMAT plans with only 6 MV arc(s). Additionally, the high skin doses in some cases (81% of the prescription dose) suggest that in hypofractionated SRS/SRT treatments, the carbon fiber couch effects on skin doses need to be evaluated when arc delivery is considered as a treatment option.
Purpose:
GRID therapy is an effective treatment for bulky tumors. Linear accelerator (Linac)-produced photon beams collimated through blocks or multi-leaf collimators (MLCs) are the most common methods used to deliver this therapy. Utilizing the newest proton delivery method of pencil beam scanning (PBS) can further improve the efficacy of GRID therapy. In this study, we developed a method of delivering GRID therapy using proton PBS, evaluated the dosimetry of this novel technique and applied this method in two clinical cases.
Materials/methods:
In the feasibility study phase, a single PBS proton beam was optimized to heterogeneously irradiate a shallow 20x20x12 cm3 target volume centered at a 6 cm depth in a water phantom. The beam was constrained to have an identical spot pattern in all layers, creating a "beamlet" at each spot position. Another GRID treatment using PBS was also performed on a deep 15x15x8 cm3 target volume centered at a 14 cm depth in a water phantom. Dosimetric parameters of both PBS dose distributions were compared with typical photon GRID dose distributions. In the next phase, four patients have been treated at our center with this proton GRID technique. The planning, dosimetry and measurements for two representative patients are reported.
Results:
For the shallow phantom target, the depth-dose curve of the PBS plan was uniform within the target (variation <5%) and dropped quickly beyond the target (50% at 12.9 cm and 0.5% at 14 cm). The lateral profiles of the PBS plan were comparable to those of photon GRID in terms of valley-to-peak ratios. For the deep phantom target, the PBS plan provided smaller valley-to-peak ratios than the photon GRID technique. Pre-treatment dose verification QA showed close agreement between the measurements and the plan (pass rate >95% with a gamma index criterion of 3%/3 mm). Patients tolerated the treatment well without significant skin toxicity (radiation dermatitis grade ≤1).
Conclusions:
Proton GRID therapy using a PBS delivery method was successfully developed and implemented clinically. Proton GRID therapy offers many advantages over photon GRID techniques. The use of protons provides a more uniform beamlet dose within the tumor and spares normal tissues located beyond the tumor. This new PBS method will also reduce the dose to proximal organs when treating a deep-seated tumor. This article is protected by copyright. All rights reserved.
Purpose: The clinical efficacy of Grid therapy has been examined by several investigators. In this project, the hole diameter and hole spacing in Grid blocks were examined to determine the optimum parameters that give a therapeutic advantage.
Methods: The evaluations were performed using Monte Carlo (MC) simulation and commonly used radiobiological models. The Geant4 MC code was used to simulate the dose distributions for 25 different Grid blocks with different hole diameters and center-to-center spacing. The therapeutic parameters of these blocks, namely, the therapeutic ratio (TR) and geometrical sparing factor (GSF) were calculated using two different radiobiological models, including the linear quadratic and Hug–Kellerer models. In addition, the ratio of the open to blocked area (ROTBA) is also used as a geometrical parameter for each block design. Comparisons of the TR, GSF, and ROTBA for all of the blocks were used to derive the parameters for an optimum Grid block with the maximum TR, minimum GSF, and optimal ROTBA. A sample of the optimum Grid block was fabricated at our institution. Dosimetric characteristics of this Grid block were measured using an ionization chamber in water phantom, Gafchromic film, and thermoluminescent dosimeters in Solid WaterTM phantom materials.
Results: The results of these investigations indicated that Grid blocks with hole diameters between 1.00 and 1.25 cm and spacing of 1.7 or 1.8 cm have optimal therapeutic parameters (TR > 1.3 and GSF~0.90). The measured dosimetric characteristics of the optimum Grid blocks including dose profiles, percentage depth dose, dose output factor (cGy/MU), and valley-to-peak ratio were in good agreement (±5%) with the simulated data.
Conclusion: In summary, using MC-based dosimetry, two radiobiological models, and previously published clinical data, we have introduced a method to design a Grid block with optimum therapeutic response. The simulated data were reproduced by experimental data.
Purpose:
Spatially fractionated radiotherapy is a strategy to overcome the main limitation of radiotherapy, i.e., the restrained normal tissue tolerances. A well-known example is Grid Therapy, which is currently performed at some hospitals using megavoltage photon beams delivered by Linacs. Grid Therapy has been successfully used in the management of bulky abdominal tumors with low toxicity. The aim of this work is to evaluate whether an improvement in therapeutic index in Grid Therapy can be obtained by implementing it in a flattening filter-free (FFF) Linac. The rationale behind is that the removal of the flattening filter shifts the beam energy spectrum towards lower energies and increase the photon fluence. Lower energies result in a reduction of lateral scattering and thus, to higher peak-to-valley dose ratios (PVDRs) in normal tissues. In addition, the gain in fluence might allow using smaller beams leading a more efficient exploitation of dose-volume effects, and consequently, a better normal tissue sparing.
Methods:
Monte Carlo simulations were used to evaluate realistic dose distributions considering a 6 MV FFF photon beam from a standard medical electron Linac and a cerrobend mechanical collimator in different configurations: grid sizes of 0.3×0.3 cm(2) , 0.5×0.5 cm(2) , and 1×1 cm(2) and a corresponding center-to-center (ctc) distance of 0.6, 1 and 2 cm, respectively (total field size of 10×10 cm(2) ). As figure of merit, peak doses in depth, PVDRs, output factors (OFs) and penumbra values were assessed.
Results:
Dose at the entrance is slightly higher than in conventional Grid Therapy. However, it is compensated by the large PVDR obtained at the entrance, reaching a maximum of 35 for a grid size of 1×1 cm(2) . Indeed, this grid size leads to very high PVDR values at all depths (≥ 10), which are much higher than in standard Grid Therapy. This may be beneficial for normal tissues but detrimental for tumor control, where a lower PVDR might be requested. In that case, higher valley doses in the tumor could be achieved by using an interlaced approach and/or adapting the ctc distance. The smallest grid size (0.3×0.3 cm(2) ) leads to low PVDR at all depths, comparable to standard Grid Therapy. However, the use of very thin beams might increase the normal tissue tolerances with respect to the grid size commonly used (1×1 cm(2) ). The gain in fluence provided by FFF implies that the important OF reduction (0.6) will not increase treatment time. Finally, the intermediate configuration (0.5×0.5 cm(2) ) provides high PVDR in the first 5 cm, and comparable PVDR to previous Grid Therapy works at depth. Therefore, this configuration might allow increasing the normal tissue tolerances with respect to Grid Therapy thanks to the higher PVDR and thinner beams, while a similar tumor control could be expected.
Conclusions:
The implementation of Grid Therapy in an FFF photon beam from medical Linac might lead to an improvement of the therapeutic index. Among the cases evaluated, a grid size of 0.5×0.5 cm(2) (1-cm-ctc) is the most advantageous configuration from the physics point of view. Radiobiological experiments are needed to fully explore this new avenue and to confirm our results. This article is protected by copyright. All rights reserved.
Conventional radiotherapy remains to be one of the most useful treatments for cancer, but it is not the best strategy to maximize the effects on the tumors and minimize the damage to the surrounding tissues because of its physical and radiobiological characteristics. Synchrotrons represent a unique method for an innovative and-cancer treatment due to the physical features (i.e. high fluence, tunable and collimated) of X-ray induced by synchrotron, so photon activation therapy and microbeam radiation treatment have been developed, but it is very imperative to understand the radiobiological mechanism of synchrotron radiation before we could transfer the strategy into the clinic. This paper is to summary the results of in vitro and in vivo experiments with synchrotron radiation, and review the advances of molecular and cellular radiobiological mechanism involved in synchrotron radiation. Since Shanghai Synchrotron Radiation Facility (SSRF) has produced the first synchrotron, it will provide the unique opportunity for the study of radiobiological effects of synchrotron radiation.
In this paper, we present the dosimetric characteristics of a commercially‐produced universal GRID block for spatially fractioned radiation therapy. The dosimetric properties of the GRID block were evaluated. Ionization chamber and film measurements using both Kodak EDR2 and Gafchromic EBT film were performed in a solid water phantom to determine the relative output of the GRID block as well as its spatial dosimetric characteristics. The surface dose under the block and at the openings was measured using ultra thin TLDs. After introducing the GRID block into the treatment planning system, a treatment plan was created using the GRID block and also by creating a GRID pattern using the multi‐leaf collimator. The percent depth doses measured with film showed that there is a shift of the dmax towards shallower depths for both energies (6 MV and 18 MV) under investigation. It was observed that the skin dose at the GRID openings was higher than the corresponding open field by a factor as high as 50% for both photon energies. The profiles showed the transmission under the block was in the order of 15–20% for 6 MV and 30% for 18 MV. The MUs calculated for a real patient using the block were about 80% less than the corresponding MUs for the same plan using the multileaf collimator to define the GRID. Based on this investigation, this brass GRID compensator is a viable alternative to other solid compensators or MLC‐based fields currently in use. Its ease of creation and use give it decided advantages. Its ability to be created once and used for multiple patients (by varying the collimation of the linear accelerator jaws) makes it attractive from a cost perspective. We believe this compensator can be put to clinical use, and will allow more centers to offer GRID therapy to their patients.
PACS number: 87.53.Mr
To present results and acute toxicity in 14 patients with bulky (>or=6 cm) tumors from locally advanced squamous cell carcinoma of the head and neck who received spatially fractionated radiotherapy (GRID) therapy to the bulky mass followed by concomitant chemoradiotherapy using simultaneous integrated boost intensity-modulated radiotherapy (SIB-IMRT).
GRID therapy to the GTV was delivered by creating one treatment field with a checkerboard pattern composed of open-closed areas using a multileaf collimator. The GRID prescription was 20 Gy in one fraction. Chemotherapy started the day of GRID therapy and continued throughout the course of SIB-IMRT. The SIB-IMRT prescription was 66, 60, and 54 Gy to the planning target volume (PTV), intermediate-risk PTV, and low-risk PTV, respectively, in 30 fractions.
With a median follow-up of 19.5 months (range, 2-38 months), the overall control rate of the GRID gross tumor volume was 79% (11 of 14). The most common acute skin and mucosal toxicities were Grade 3 and 2, respectively.
For the treatment of locally advanced neck squamous cell carcinoma of the head and neck, GRID followed by chemotherapy and SIB-IMRT is well tolerated and yields encouraging clinical and pathologic responses, with similar acute toxicity profiles as in patients receiving chemoradiotherapy without GRID.
The purpose of this work is to evaluate the modeling of carbon fiber couch attenuation properties with a commercial treatment planning system (TPS, Pinnacle3, v8.0d). A carbon fiber couch (Brain-Lab) was incorporated into the TPS by automatic contouring of all transverse CT slices. The couch shape and dimensions were set according to the vendor specifications. The couch composition was realized by assigning appropriate densities to the delineated contours. The couch modeling by the TPS was validated by absolute dosimetric measurements. A phantom consisting of several solid water slabs was CT scanned, the CT data set was imported into the TPS, and the carbon fiber couch was auto-contoured. Open (unblocked) field plans for different gantry angles and field sizes were generated. The doses to a point at 3 cm depth, placed at the linac isocenter, were computed. The phantom was irradiated according to the dose calculation setup and doses were measured with an ion chamber. In addition, percent depth dose (PDD) curves were computed as well as measured with radiographic film. The calculated and measured doses, transmissions, and PDDs were cross-compared. Doses for several posterior fields (0 degree, 30 degrees, 50 degrees, 75 degrees, 83 degrees) were calculated for 6 and 18 MV photon beams. For model validation a nominal field size of 10 x 10 cm2 was chosen and 100 MU were delivered for each portal. The largest difference between computed and measured doses for those posterior fields was within 1.7%. A comparison between computed and measured transmissions for the aforementioned fields was performed and the results were found to agree within 1.1%. The differences between computed and measured doses for different field sizes, ranging from 5 x 5 cm2 to 25 x 25 cm2 in 5 cm increments, were within 2%. Measured and computed PDD curves with and without the couch agree from the surface up to 30 cm depth. The PDDs indicate a surface dose increase resulting from the carbon fiber couch field modification. The carbon fiber couch attenuation for individual posterior oblique fields (75 degrees) can be in excess of 8% depending on the beam energy and field size. When the couch is contoured in Pinnacle3 its attenuation properties are modeled to within 1.7% with respect to measurements. These results demonstrate that appropriate contouring together with relevant density information for the contours is sufficient for adequate modeling of carbon fiber supporting devices by modern commercial treatment planning systems.
The tissue-sparing effect of parallel, thin (narrower than 100 microm) synchrotron-generated X-ray planar beams (microbeams) in healthy tissues including the central nervous system (CNS) is known since early 1990 s. This, together with a remarkable preferential tumoricidal effect of such beam arrays observed at high doses, has been the basis for labeling the method microbeam radiation therapy (MRT). Recent studies showed that beams as thick as 0.68 mm ("thick microbeams") retain part of their sparing effect in the rat's CNS, and that two such orthogonal microbeams arrays can be interlaced to produce an unsegmented field at the target, thus producing focal targeting. We measured the half-value layer (HVL) of our 120-keV median-energy beam in water phantoms, and we irradiated stereotactically bis acrylamide nitrogen gelatin (BANG)-gel-filled phantoms, including one containing a human skull, with interlaced microbeams and imaged them with MRI. A 43-mm water HVL resulted, together with an adequately large peak-to-valley ratio of the microbeams' three-dimensional dose distribution in the vicinity of the 20 mm x 20 mm x 20 mm target deep into the skull. Furthermore, the 80-20% dose fall off was a fraction of a millimeter as predicted by Monte Carlo simulations. We conclude that clinical MRT will benefit from the use of higher beam energies than those used here, although the current energy could serve certain neurosurgical applications. Furthermore, thick microbeams particularly when interlaced present some advantages over thin microbeams in that they allow the use of higher beam energies and they could conceivably be implemented with high power orthovoltage X-ray tubes.
The relationship between the tumor control frequency and the radiation dose for radiation treatment of a solid Ehrlich ascites carcinoma by open field technique and sieve technique is presented.
The TCD50 (dose to control half of the tumor) was found to be higher for the sieve than for the open field method. The TCD50 (70 days) was 4,751 rads by the sieve technique and 3,871 rads by the open field technique. The ratio of TCD50 sieve/open field was 1.23.
The reaction of normal tissue surrounding the tumor to a dose producing a given tumor control frequency was markedly less if that dose were administered through a sieve than if through an open field. The radiation dose which induced skin retraction was > 1.8 times higher by the sieve techniques than by the open field techniques.
Purpose:
With the advent of megavoltage radiation, the concept of spatially-fractionated (SFR) radiation has been abandoned for the last several decades; yet, historically, it has been proven to be safe and effective in delivering large cumulative doses (> 100 Gy) of radiation in the treatment of cancer. SFR radiation has been adapted to megavoltage beams using a specially constructed grid. This study evaluates the toxicity and effectiveness of this approach in treatment of advanced and bulky cancers.
Methods and materials:
From January 1995 through March 1998, 71 patients with advanced bulky tumors (tumor sizes > 8 cm) were treated with SFR high-dose external beam megavoltage radiation using a GRID technique. Sixteen patients received GRID treatments to multiple sites and a total of 87 sites were irradiated. A 50:50 GRID (open to closed area) was utilized, and a single dose of 1,000-2,000 cGy (median 1,500 cGy) to Dmax was delivered utilizing 6 MV photons. Sixty-three patients received high-dose GRID therapy for palliation (pain, mass, bleeding, or dyspnea). In 8 patients, GRID therapy was given as part of a definitive treatment combined with conventionally-fractionated external beam irradiation (dose range 5,000-7,000 cGy) followed by subsequent surgery. Forty-seven patients were treated with GRID radiation followed by additional fractionated external beam irradiation, and 14 patients were treated with GRID alone. Thirty-one treatments were delivered to the abdomen and pelvis, 30 to the head and neck region, 15 to the thorax, and 11 to the extremities.
Results:
For palliative treatments, a 78% response rate was observed for pain, including a complete response (CR) of 19.5%, and a partial response (PR) of 58.5% in these large bulky tumors. A 72.5% response rate was observed for mass effect (CR 14.6%, PR 52.9%). The response rate observed for bleeding was 100% (50% CR, 50% PR) and for dyspnea, a 60% PR rate only. A relatively higher response rate (CR 23.3%, PR 60%) was observed in patients who received GRID treatment in the head and neck area. No grade 3 late skin, subcutaneous, mucosal, GI, or CNS complications were observed in any patient in spite of these high doses. In the 8 patients who received GRID treatment for definitive treatment, a clinical CR was observed in 5 patients (62.5%) and a pathological complete response was confirmed in the operative specimen in 4 patients (50%).
Conclusion:
The efficacy and safety of using a large fraction of SFR radiation was confirmed by this study and substantiates our earlier results. In selected patients with bulky tumors (> 8 cm), SFR radiation can be combined with fractionated external beam irradiation to yield improved local control of disease, both for palliation and selective definitive treatment, especially where conventional treatment alone has a limited chance of success.
Background: Grid radiation therapy, using the megavoltage X-ray beam, has been proven to be an effective method for management of large and bulky malignant tumors. This treatment modality is also known as Specially Fractionated Radiation Therapy (SFRT). In this treatment technique a grid block converted the open radiation field into a series of pencil beams. Dosimetric characteristics of an external beam grid radiation field have been investigated using experimental and Monte Carlo simulation technique. Materials and methods: Dose distributions (%DD as well as the beam profiles) of a grid radiation field have been determined using experimental and Monte Carlo simulation technique, for 6- and 18 MV X-ray beams from a Varian Clinics 2100C/D. The measurements were performed using LiF TLD and film in Solid Water phantom Material. Moreover, the MCNP Monte Carlo code was utilized to calculate the dose distribution in the grid radiation field in the same phantom material. The results of the experimental data were compared to the theoretical values, to validate this technique. Upon the agreement between the two techniques, dose distributions can be calculated for the grid field with different patterns and sizes of holes, in order to find an optimal design of the grid block. Results: The results of dose profiles for 6 MV X-ray beams obtained with the Monte Carlo simulation technique was in good agreement with the measured data. In addition, the 3D dose distribution of the grid field generated by the Monte Carlo simulation gave more detailed information about the dose pattern of the grid. Conclusion: The grid block can be used as a boost for treatment of bulky tumors. The Monte Carlo simulation technique can be utilized to optimize the pattern, size and spacing between the holes, for optimal clinical results.
In the era of orthovoltage radiation, multiple nonconfluent pencil beam radiation (GRID) therapy was utilized to minimize superficial normal tissue damage while delivering tumorcidal doses at specified depths in tissues. The success of GRID therapy was based on the fact that small volumes of tissues could tolerate high doses of radiation. Since the development of megavoltage radiation and skin sparing, GRID therapy has been abandoned. In a pilot study, the authors adapted the principles of GRID therapy to megavoltage photon beams to treat patients with massive tumors or recurrent tumors after tolerance doses of radiation. Twenty-two patients have been entered in the study. All patients were symptomatic and had exhaustive conventional surgery, chemotherapy, and radiotherapy approaches to treatment. A 50:50 GRID (open to closed areas) was utilized, and a prescribed dose of 1000 to 1500 cGy to the open areas was given using a single photon field. In four patients, a second GRID treated was delivered at a split course interval of 4 weeks. The follow-up in these patients ranges from 1 month to 18 months. The results of treatment have been remarkable with 20 of 22 patients achieving dramatic relief of severe symptoms, and several patients showing significant objective regression. No acute effects have been observed, including those patients having large volumes of the abdomen irradiated. No unusual skin or subcutaneous early or late damage has been observed in follow-up.
The stem-cell depletion hypothesis postulates that there exists for each organ a limiting critical volume, which can be repopulated by a single surviving stem cell and for which damage can be repaired by repopulation (Yaes & Kalend, 1988; Yaes et al, 1988). When the critical volume is totally depleted of stem cells, irreversible damage occurs. Certain organs such as the spinal cord yield very interesting experimental results, where the frequency of transverse myelitis has varied with the minimal critical length of cord irradiated (Hopewell et al, 1987; van der Kogel, 1987; Hopewell & van der Kogel, 1988). The critical length of spinal cord has been determined to be approximately 2–5 mm in WAG/Rij rats and <4mm in Sprague-Dawley rats. Both sets of data indicated a critical length of spinal cord that must be irradiated to cause myelitis and that lies between 2 and 5 mm for the rat. Similar predictions have been made using a model based on functional subunits (Withers & Taylor, 1988; Withers et al, 1988).
Grid therapy is a technique used to deliver a high dose of radiation (15-20 Gy) in a single fraction to many small volumes within a large treatment field. This treatment modality is used for the palliative treatment of large, deeply seated tumors, which have either been treated to tolerance with conventional radiation, or, due to massive tumor bulk, would most likely not benefit from a conventional course of radiation therapy. As the dose distribution from megavoltage grid therapy differs significantly from that of conventional radiation therapy (i.e., many large dose gradients exist within the tumor volume), we have measured various dosimetric properties inherent in this unique treatment modality.
The grid is a 16 x 16 array of 1-cm diameter holes in a 7-cm thick piece of custom blocking material. The ratio of shielded to open surface area is 1:1. Depth dose, valley-to-peak ratios, and output factors for this square array grid were measured in a water phantom for several field sizes, as well as for a 1-cm diameter narrow beam using 6 MV and 25 MV photon beams.
The depth dose curves for the grid fields lie between those for an open portal and a narrow beam. For the 6-MV beam at dmax, the ratios of the doses delivered to the center of the shielded regions to that under the center of the holes, expressed as valley-to-peak ratios, range from 15 to 40%. At 10 cm, the ratios increase to between 25 and 45%. At 25 MV at both dmax and 10 cm, the valley-to-peak ratios are between 40 and 60%. The output factors, 0.89 for 6 MV and 0.77 for 25 MV, do not depend on field size.
Megavoltage grid therapy is a unique treatment modality where the dose is delivered differentially to a large volume in one fraction. Characterization of the dosimetric properties has allowed clinical implementation of the grid.
When all sites and types of cancer are included, the cure rate from currently accepted methods of treatment does not exceed 25 per cent. This means that about 75 per cent of all cancer victims must be considered incurable and can be treated only palliatively for prolongation of useful life or relief of symptoms, or both. The value of radiotherapy as a palliative agent is not universally appreciated. There is often a lack of interest in the hopeless or incurable patient and either no active treatment is given or only meaningless token therapy. Recently, renewed interest has been displayed in the use of grids for roentgen therapy of advanced cancers. Grids have been used in roentgen therapy since 1909, when Alban Kohler (1) made his first report. Later, in 1933, Liberson (2) suggested the use of a metallic lead grid and worked out the biological and physical aspects which are, for the most part, valid to this day. Others, including Haring (3), Grynkraut (4), and Jolles (5) have also suggested the use of gri...
To decrease the effects of x-rays on the skin when large fields are used to reach a deep-seated cancer, Alban Kohler (1) in 1909 had devised what has been termed “fractionation of the x-ray dosage in space.” This was done by placing on the skin a steel wire net, the openings of which were 2 mm. wide and the wire proper 1 mm. in thickness. Kohler administered massive doses, ten to twenty times the total dose usually given at that time through a conventional open field. Healing of the skin was initiated in and extended from the surrounding relatively unscathed normal skin. In experimental radiobiology in 1931 (2) and 1938 (3a), doses of 25,000 r and 120,000 r without a grid were found to be inhibitory and lethal respectively to the proliferation of normal tissue in vitro, the cancericidal dose in vivo being 24,000 r in air (3b). In the above case, the optimum dose without a grid enters the tumor-bearing area, while in the grid method only 40 per cent of the 24,000 r in air, or 9,600 r in air, enters the tum...
In the present work, we used model calculations of cell survival to compare the effects of single fraction high-dose grid therapy with those of uniform dose delivery on tumor and normal tissues.
The grid consisted of a hexagonal pattern of divergent holes in a Cerrobend block. A linear-quadratic model was used to find the surviving fraction of tumor and normal tissue cells after high-dose irradiation. Equivalent uniform doses were determined according to the tumor cell kill. The ratio of the normal tissue surviving fraction under grid irradiation to that obtained under equivalent uniform dose irradiation was taken as a measure of therapeutic gain.
The therapeutic ratio varied from 0.80 to 13.22 for the range of cell sensitivities investigated, with single fraction doses of 10.0-20.0 Gy. Optimization studies showed no significant dependence of therapeutic gain on hole spacing.
With high, single-fraction doses, grid irradiation revealed a therapeutic advantage over uniform dose irradiation whenever the tumor and surrounding normal tissues cells were equally radiosensitive, or, particularly, if the tumor cells were more radioresistant than the normal cells. The therapeutic gain did not appear to be a strong function of grid design.
Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, combined with conventional radiation therapy, has been demonstrated using a prototype GRID block [Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114-118 (1990)]. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radiation Product Design, Inc. (Albertive, MN). This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investigated for a Varian 21EX linear accelerator (Varian Associates, Palo Alto, CA). These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with alpha/beta ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented.
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A. S. Meigooni, K. Dou, N. J. Soleimani-Meigooni, M. Gnaster, S. B.
Semi‐empirical calculation of dose distributions for high energy photon beam grid therapy
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Stemcell deletion and GRID therapy Therapeutic advantage of GRID irradiation for large single fractions
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Palliative treatment of advanced cancer using multiple nonconfluent pencil beam radiation
- Mohiuddin