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Changes in beam profiles for nominal energies of 4 and 10 MV flattened beams. Bending magnet current values were changed by-10% and +10% at the depth of maximum dose (d max ) and at a depth of 10 cm (d 10 ) in water. Note that the off-axis distance of the minimum and maximum change with energy and depth.
Source publication
In extension of a previous study, we compared several photon beam energy metrics to determine which was the most sensitive to energy change; in addition to those, we accounted for both the sensitivity of each metric and the uncertainty in determining that metric for both traditional flattening filter (FF) beams (4, 6, 8, and 10 MV) and for flatteni...
Context in source publication
Context 1
... nominal energy as a function of BMI in water and with the ICA (Table 2). The Flat increased with energy near d max but decreased with energy near 10 cm depth (Fig. 2(b), Table 3). This reversal in trend is explained by the fact that flat is not defined at a fixed off-axis distance, and the position of the min and max change with energy and depth (Fig. ...
Citations
... These magnets are designed to select a narrow mono-energetic electron beam energy with a small standard deviation. 24 Since the relationship between bending magnet current and electron beam energy is linear, [25][26][27] this provides a convenient method for varying the linear accelerator beam energy in a quantifiable manner and therefore a measurement of the primary electron energy. ...
... This current-energy relationship was then used to determine the electron beam energy 26,27 for beams of intermediate energy. With the bending magnet calibrated, any energy between the minimum and maximum machine energy can be set by adjusting the bending magnet current to the desired energy, and then configuring the accelerator beam parameters (injector current and PFN setting) so that the machine dose rate is maximized, which ensures that the electron beam strikes the target at a known energy. ...
Background
The recent trend toward 10 MV for volumetric radiotherapy treatment such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), and stereotactic ablative body radiotherapy (SABR) introduces photoneutron production, with implications for non‐therapeutic patient dose and additional shielding requirements for treatment room design. The sharply nonlinear drop‐off in photoneutron production below 10 MV to negligible at 6 MV has scarcely been characterized quantitatively, yet can elucidate important practical insights.
Purpose
To measure photoneutron yields in a medical linac at 8 MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6 MV while reducing photoneutron production which is present at 10 MV.
Methods
A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15 MV by calibrating the bending magnet and adjusting other beam generation parameters. Neutron dose within the treatment room was measured using an Anderson‐Braun type detector over a continuum of intermediate energies.
Results
The photoneutron production for energies below 10 MV was measured, adding to data that is otherwise scarce in the literature. Our results are consistent with previously published results for neutron yield. We found that the photoneutron production at 8 MV was about 1/10 of the value at 10 MV, and about 10 times higher than detector background at 6 MV.
Conclusions
Photoneutron production drops off below 10 MV, but is still present at 8 MV. An 8 MV beam is more penetrating than a 6 MV beam, and may offer a suitable tradeoff for modern radiotherapy techniques such as VMAT, SRS, and SABR. Further studies are needed to better understand the impact on treatment plan quality between 8 and 10 MV beams considering the benefits to facility requirements and non‐therapeutic patient dose.
... Likewise, beam profile specifications included flatness, symmetry, and penumbra. The flatness and symmetry which, define beam uniformity were determined within the central 80% of the full-width-at-half maximum (FWHM) of the processed beam profiles [12][13][14]. ...
... We found that the beam commissioning data presented in this report for the Varian 21EX linac compare favourably with data documented for other Varian linacs [13,14,21]. Photon PDDs agreed within 1% of the average of several linacs of the same type at depths between 5 and 20 cm, which are consistent with the data used by the manufacturer for acceptance testing. ...
With increasing cancer incidence in Africa, a number of Sub-Saharan African countries have started implementing radiotherapy programs based on linear accelerator (linac) technology. This work summarizes the commissioning experience of a commercially available medical linac installed in a resource-limited oncology centre in Cameroon for the delivery of high-quality radiation treatments to cancer patients in central Africa. Cameroon is a central African country in Sub-Saharan Africa. Using a 2D water phantom and various ionization chambers, we measured commissioning data for a medical linac with 6X and 18X photon beams, and five electron energies ranging from 6-20 MeV. Relative measurements included percent depth doses (PDDs), beam profiles, scatter factors, wedge factors, and electron cone factors. Absolute calibrations of the beam energies were performed using the American Association of Physicist in Medicine Task Group Report 51. Accurate calibrations were checked by irradiating the mailed thermoluminescent dosimeters service offered by MD Anderson Cancer Center. Photon PDDs agreed within 1% of the average of several linacs of the same type at depths between 5 and 20 cm, which are consistent with the data used by the manufacturer for acceptance testing. For electrons, the agreement was within 2 mm for R 50 , R 90 , R p , and d max. Symmetry and flatness for all photon and electron beams were within 2% for various fields. All absolute calibrations met the MD Anderson Cancer Center criteria within 3%. This work presents the successful implementation and modernization of a radiotherapy program based on linac technology in the central African sub-region in Sub-Saharan Africa. As the first operational medical linac in the sub-region, the commissioning data can provide comparison data to other linacs in the future to ensure high-quality of machine commissioning for clinical use.
... The required routine linear accelerator (linac) quality assurance (QA) tests and their tolerances and frequencies are stipulated in best practice documents such as the American Association of Physicists in Medicine Task Group 142 report 1 and Medical Physics Practice Guideline 8.a. 2 Both documents require routine QA testing of the beam dose profile shape and the photon beam quality for which a number of publications have suggested that metrics such as flatness and off -axis-ratio can be used as an effective measure. [3][4][5] The amorphous silicon electronic portal imaging device (EPID) was designed for patient positioning applications. For such applications, the image nonuniformities caused by the patient anatomy in the beam are of interest. ...
Purpose
Calibration of a radiotherapy electronic portal imaging device (EPID) using the pixel‐sensitivity‐map (PSM) in place of the flood field correction improves the utility of the EPID for quality assurance applications. Multiple methods are available for determining the PSM and this study provides an evaluation to inform on which is superior.
Methods
Three different empirical methods (“Calvary Mater Newcastle” [CMN], “Varian,” and “WashU”) and a Monte Carlo‐based method of PSM determination were investigated on a single Varian TrueBeam STx linear accelerator (linac) with an aS1200 EPID panel. PSM measurements were performed for each empirical method three successive times using the 6 MV beam. The resulting PSM from each method was compared to the Monte Carlo method as a reference using 2D percentage deviation maps and histograms plus crossplane profiles. The repeatability of generated PSMs was also assessed via 2D standard deviation (SD) maps and histograms. Additionally, the Beam‐Response generated by removal of the PSM from a raw EPID image for each method was visually contrasted. Finally, the practicality of each method was assessed qualitatively and via the measured time required to acquire and export the required images.
Results
The median pixel‐by‐pixel percentage deviation between each of the empirical PSM methods and the Monte Carlo PSM was ‐0.36%, 0.24%, and 0.74% for the CMN, Varian, and WashU methods, respectively. Ninety‐five percent of pixels were found to be repeatable to within ‐0.21%, 0.08%, 0.19%, and 0.35% (1 SD) for the CMN, Monte Carlo, Varian, and WashU methods, respectively. The WashU method was found to be quickest for data acquisition and export and the CMN the slowest.
Conclusion
For the first time four methods of generating the EPID PSM have been compared in detail and strengths and weaknesses of each method have been identified. All methods are considered likely to be clinically acceptable and with similar practical requirements.
... A recent study demonstrated that the Halcyon platform can be validated with an ionization chamber array (ICA) and a 1D water scanner (1DS) without the need for a 3D water scanning system. 1 The commissioning verification was based on the AAPM Medical Physics Practice Guideline for Commissioning and QA of External Beam Planning Systems (MPPG5.a). 2 The diagonal normalized flatness (F DN ), which is calculated from open beam profiles measured with an ICA, was verified as a metric for monitoring beam energy and was more sensitive and reproducible than the traditional percent depth dose (PDD) energy metric. 3,4 Another method for determining photon beam energy uses a quad wedge (QW) which consists of two pairs of copper wedge-shaped attenuators along each of the diagonal detector axes of the ICA, with the wedge pairs being symmetrically opposed and the thin sections toward the array center. 5 The energy metric from the QW is the area ratio (AR), which is defined as the cumulative of measurements from a span of detectors in the presence of wedges and normalized to the cumulative of measurements from a similar detector set on the X and Y axes (open field profiles). ...
... Without a direct means of determining how changes in magnetron current (MI) would change energy, previously determined relationships were used between changes in diagonal normalized flatness F DN and changes in energy on a TrueBeam 6-MV FFF beam. 3 Each energy metric was measured at the nominal beam energy and at five intentionally created energy changes off -10.0%, -5.0%, -2.5%, +2.5%, and +5.0% from the nominal beam energy value (0%). After beam tuning to achieve stable dose rates and symmetric beam profiles for each MI setting, the beam parameters were saved to a file and later loaded for each tuned beam, which provided reproducible measurement setups. ...
Purpose:
Establish and compare two metrics for monitoring beam energy changes in the Halcyon platform and evaluate the accuracy of these metrics across multiple Halcyon linacs.
Method:
The first energy metric is derived from the diagonal normalized flatness (FDN ), which is defined as the ratio of the average measurements at a fixed off-axis equal distance along the open profiles in two diagonals to the measurement at the central axis with an ionization chamber array (ICA). The second energy metric comes from the area ratio (AR) of the quad wedge (QW) profiles measured with the QW on the top of the ICA. Beam energy is changed by adjusting the magnetron current in a non-clinical Halcyon. With D10cm measured in water at each beam energy, the relationships between FDN or AR energy metrics to D10cm in water is established with linear regression across six energy settings. The coefficients from these regressions allow D10cm (FDN ) calculation from FDN using open profiles and D10cm (QW) calculation from AR using QW profiles.
Results:
Five Halcyon linacs from five institutions were used to evaluate the accuracy of the D10cm (FDN ) and the D10cm (QW) energy metrics by comparing to the D10cm values computed from the treatment planning system (TPS) and D10cm measured in water. For the five linacs, the D10cm (FDN ) reported by the ICA based on FDN from open profiles agreed with that calculated by TPS within -0.29 ± 0.23% and 0.61% maximum discrepancy; the D10cm (QW) reported by the QW profiles agreed with that calculated by TPS within -0.82 ± 1.27% and -2.43% maximum discrepancy.
Conclusion:
The FDN -based energy metric D10cm (FDN ) can be used for acceptance testing of beam energy, and also for the verification of energy in periodic quality assurance (QA) processes.
... In particular, central longitudinal profile at 0, positive longitudinal profile at + 3 cm and negative longitudinal profile at − 3 cm to the central longitudinal profile respectively, have been computed. Flatness [16] has also been computed both on the longitudinal profile and on a field size 6.0x30.0 cm 2 . ...
Purpose:
Due to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual-isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery technique for MR-Linac.
Material and methods:
Dual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in clinical practice has then been retrospectively re-planned with dual-isocenter technique. The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of ± 2 mm and ± 5 mm positioning errors by shifting the couch.
Results:
MLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation. 2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis. The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and 3.7% (PTV).
Conclusion:
The dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size.
... The QA radiotherapy machines is performed to confirm that the characteristics of the commissioned machines are identical to those of the tested prototype [1]. Periodic QA can be performed on a daily, weekly, monthly, or annual basis, and the machine status can be updated by comparing the measured and analyzed results against available guidelines. ...
... Several studies [1,10,12] have been performed to develop a robust method for energychange monitoring using beam flatness as a governing parameter. There exists a reason for considering the changes in beam flatness for energy-change monitoring. ...
... Accordingly, it is important for medical physicist to understand the sensitivity of the flatness change when monitoring energy changes using the Daily QA3 system. Moreover, Gao et al. [1] proposed the off-axis ratio-based (OAR) metrics to be used in cases involving a flattening-filterfree (FFF) beam. However, the scope of this study is limited to monitoring energy changes in the 6 and 10 MV FF beams only using the Daily QA3 system. ...
This study evaluates the changes occurring in the X-ray energy of a linear accelerator (LINAC) using a Daily QA3 detector system. This is accomplished by comparing the Daily QA3 results against those obtained using a water phantom. The X-energy levels of a LINAC were monitored over a duration of 1 month using the Daily QA3 system. Moreover, to account for the uncertainty, the reproducibility of the Daily QA3 ionization-chamber results was assessed by performing repeated measurements (12 per day). Subsequently, the energy-monitoring results were compared with the energy-change results calculated using the water-phantom percentage depth dose (PDD) ratio. As observed, the 6- and 10-MV beams experienced average daily energy-level changes of (-0.30 ± 0.32)% and (0.05 ± 0.38)%, respectively, during repeated measurements. The corresponding energy changes equaled (-0.30 ± 0.55)% and (-0.05 ± 0.48)%, respectively, when considering the measurement uncertainty. The Daily QA3 measurements performed at 6 MV demonstrated a variation of (2.15 ± 0.81)% (i.e., up to 3%). Meanwhile, the corresponding measurements performed using a water phantom demonstrated an increase in the PDD ratio from 0.577 to 0.580 (i.e., approximately 0.5%). At 10 MV, the energy variation in the Daily QA3 measurements equaled (-0.41 ± 0.82)% (i.e., within 1.5%), whereas the corresponding water phantom PDD ratio remained constant at 0.626. These results reveal that the Daily QA3 system can be used to monitor small energy changes occurring within radiotherapy machines. This demonstrates its potential for use as a secondary system for monitoring energy changes as part of the daily quality-assurance workflow.
... Furthermore, photon beam energy constancy metric, previous studies [15][16][17] have demonstrated that an ICA can be used to measure changes in the energy, characterized by off-axis ratio (e.g., in diagonal axes, diagonal normalize flatness, F DN ) with a higher sensitivity than can be achieved with percentage depth-dose measurements. ...
Validate that a two‐dimensional (2D) ionization chamber array (ICA) combined with a double‐wedge plate (DWP) can track changes in electron beam energy well within 2.0 mms as recommended by TG‐142 for monthly quality assurance (QA). Electron beam profiles of 4–22 MeV were measured for a 25 × 25 cm2 cone using an ICA with a DWP placed on top of it along one diagonal axis. The relationship between the full width half maximum (FWHM) field size created by DWP energy degradation across the field and the depth of 50% dose in water (R50) is calibrated for a given ICA/DWP combination in beams of know energies (R50 values). Once this relationship is established, the ICA/DWP system will report the R50FWHM directly. We calibrated the ICA/DWP on a linear accelerator with energies of 6, 9, 12, 16, 20, and 22 MeV. The R50FWHM values of these beams and eight other beams with different R50 values were measured and compared with the R50 measured in water, that is, R50Water. Resolving changes of R50 up to 0.2 cm with ICA/DWP was tested by adding solid‐water to shift the energy and was verified with R50Water measurements. To check the long‐term reproducibility of ICA/DWP we measured R50FWHM on a monthly basis for a period of 3 yr. We proposed a universal calibration procedure considering the off‐axis corrections and compared calibrations and measurements on three types of linacs (Varian TrueBeam, Varian C‐series, and Elekta) with different nominal energies and R50 values. For all 38 beams on same type of linac with R50values over a range of 2–8.8 cm, the R50FWHM reported by the ICA/DWP system agreed with that measured in water within 0.01 ± 0.03 cm (mean ± 1σ) and maximum discrepancy of 0.07 cm. Long‐term reproducibility results show the ICA/DWP system to be within 0.04 cm of their baseline over 3 yr. With the universal calibration the maximum discrepancy between R50FWHM and R50Water for different types of linac reduced from 0.25 to 0.06 cm. Comparison of R50FWHM values and R50Water values and long‐term reproducibility of R50FWHM values indicates that the ICA/DWP can be used for monitoring of electron beam energy constancy well within TG‐142 recommended tolerance.
... The off-axis ratio (OAR), defined as the ratio of the average measurements at a fixed distance (e.g., 80% of the field size) along the profiles in two orthogonal axes from the beam CAX to the measurement at the CAX, is used as the beam energy metric. 6 Recent studies indicated that an OAR-based metric is more sensitive to energy changes than a PDD metric. 6,7 We also compared the OAR on the principal x and y axes from the profiles measured with ICA to those from TPS calculations and from 3DWS profiles. ...
... 6 Recent studies indicated that an OAR-based metric is more sensitive to energy changes than a PDD metric. 6,7 We also compared the OAR on the principal x and y axes from the profiles measured with ICA to those from TPS calculations and from 3DWS profiles. ...
We tested whether an ionization chamber array (ICA) and a one‐dimensional water scanner (1DS) could be used instead of a three‐dimensional water scanning system (3DWS) for acceptance testing and commissioning verification of the Varian Halcyon–Eclipse Treatment Planning System (TPS). The Halcyon linear accelerator has a single 6‐MV flattening‐filter‐free beam and a nonadjustable beam model for the TPS. Beam data were measured with a 1DS, ICA, ionization chambers, and electrometer. Acceptance testing and commissioning were done simultaneously by comparing the measured data with TPS‐calculated percent‐depth‐dose (PDD) and profiles. The ICA was used to measure profiles of various field sizes (10‐, 20‐, and 28 cm2) at depths of dmax (1.3 cm), 5‐, 10‐, and 20 cm. The 1DS was used for output factors (OFs) and PDDs. OFs were measured with 1DS for various fields (2–28 cm2) at a source‐to‐surface distance of 90 cm. All measured data were compared with TPS‐calculations. Profiles, off‐axis ratios (OAR), PDDs and OFs were also measured with a 3DWS as a secondary check. Profiles between the ICA and TPS (ICA and 3DWS) at various depths across the fields indicated that the maximum discrepancies in high‐dose and low‐dose tail were within 2% and 3%, respectively, and the maximum distance‐to‐agreement in the penumbra region was <3 mm. The largest OAR differences between ICA and TPS (ICA and 3DWS) values were 0.23% (−0.25%) for a 28 × 28 cm2 field, and the largest point‐by‐point PDD differences between 1DS and TPS (1DS and 3DWS) were −0.41% ± 0.12% (−0.32% ± 0.17%) across the fields. Both OAR and PDD showed the beam energy is well matched to the TPS model. The average ratios of 1DS‐measured OFs to the TPS (1DS to 3DWS) values were 1.000 ± 0.002 (0.999 ± 0.003). The Halcyon–Eclipse system can be accepted and commissioned without the need for a 3DWS.
... The IAEA TRS 398 protocol recommends evaluating the beam quality index based on TPR parameter for field size of 10 Â 10 cm 2 as a quotient of dose at a depth of 20 cm to dose of a depth of 10 cm (IAEA, 2004b). For determination facility, the percentage depth dose (PDD) method is introduced which is as a quotient of PDD at a depth of 20 cm to PDD of a depth of 10 cm (Song et al., 2016). The quality control improvement aims always to ensure high radiotherapy quality of cancer treatment (AAPM, 1994b). ...
The quality is essential for radiation use in medicine and the beam quality index is the basic parameter to periodically check the normal functioning of Linac head in radiotherapy treatment. It is recommended by IAEA protocols TRS-398 based on absorbed dose in water. The PDD method is used as a basis of the parameterization of the photon beam quality for predicting its variation with beam energy and field size. The objective of this work is to establish a mathematical law for predicting and checking the beam dosimetry quality index according to field size and beam energy based on IAEA TRS-398 protocol.
For an easier and more reliable procedure determination of the beam quality based on TRS 398, two empirical laws were therefore established with an accuracy better than 2%. They can serve the basic floor to medical physicist to verify and to control the dosimetry output quality at arbitrary field size. Our findings aim to facilitate the dosimetry quality control that set in use according to current conditions for checking out the radiotherapy efficiency and the safety inside the treatment room.
... The beam profiles specifications, which are beam flatness, beam symmetry and penumbra width, are given in Table 3. According to manufacture specifications and International Electrotechnical Commission (IEC) [12] the limits of flatness, which is the variation of dose relative to the central axis over the central 80% of the field size at a 10 cm depth in a plane perpendicular to the central axis [14], is ±3%, symmetry is ±2%. From Table 3 Elekta Synergy platform Linac meets the manufacturer's and ICE specifications on beam flatness and symmetry. ...
