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Title: SU-F-P-49: Comparison of Mapcheck 2 Commission for Photon and Electron Beams

Abstract

Purpose: We will investigate the performance variation of the MapCheck2 detector array with different array calibration and dose calibration pairs from different radiation therapy machine. Methods: A MapCheck2 detector array was calibrated on 3 Elekta accelerators with different energy of photon (6 MV, 10 MV, 15 MV and 18 MV) and electron (6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV and 20 MeV) beams. Dose calibration was conducted by referring a water phantom measurement following TG-51 protocol and commission data for each accelerator. A 10 cm × 10 cm beam was measured. This measured map was morphed by applying different calibration pairs. Then the difference was quantified by comparing the doses and similarity using gamma analysis of criteria (0.5 %, 0 mm). Profile variation was evaluated on a same dataset with different calibration pairs. The passing rate of an IMRT QA planar dose was calculated by using 3 mm and 3% criteria and compared with respect to each calibration pairs. Results: In this study, a dose variation up to 0.67% for matched photons and 1.0% for electron beams is observed. Differences of flatness and symmetry can be as high as 1% and 0.7% respectively. Gamma analysis shows amore » passing rate ranging from 34% to 85% for the standard 10 × 10 cm field. Conclusion: Our work demonstrated that a customized array calibration and dose calibration for each machine is preferred to fulfill a high standard patient QA task.« less

Authors:
; ; ;  [1]
  1. University of Mississippi Med. Center, Jackson, MS (United States)
Publication Date:
OSTI Identifier:
22626719
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; ACCELERATORS; CALIBRATION; DATASETS; ELECTRON BEAMS; PATIENTS; PERFORMANCE; PHANTOMS; RADIATION DOSES; RADIOTHERAPY; SYMMETRY

Citation Formats

Lu, J, Yang, C, Morris, B, and Markovich, A. SU-F-P-49: Comparison of Mapcheck 2 Commission for Photon and Electron Beams. United States: N. p., 2016. Web. doi:10.1118/1.4955756.
Lu, J, Yang, C, Morris, B, & Markovich, A. SU-F-P-49: Comparison of Mapcheck 2 Commission for Photon and Electron Beams. United States. doi:10.1118/1.4955756.
Lu, J, Yang, C, Morris, B, and Markovich, A. 2016. "SU-F-P-49: Comparison of Mapcheck 2 Commission for Photon and Electron Beams". United States. doi:10.1118/1.4955756.
@article{osti_22626719,
title = {SU-F-P-49: Comparison of Mapcheck 2 Commission for Photon and Electron Beams},
author = {Lu, J and Yang, C and Morris, B and Markovich, A},
abstractNote = {Purpose: We will investigate the performance variation of the MapCheck2 detector array with different array calibration and dose calibration pairs from different radiation therapy machine. Methods: A MapCheck2 detector array was calibrated on 3 Elekta accelerators with different energy of photon (6 MV, 10 MV, 15 MV and 18 MV) and electron (6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV and 20 MeV) beams. Dose calibration was conducted by referring a water phantom measurement following TG-51 protocol and commission data for each accelerator. A 10 cm × 10 cm beam was measured. This measured map was morphed by applying different calibration pairs. Then the difference was quantified by comparing the doses and similarity using gamma analysis of criteria (0.5 %, 0 mm). Profile variation was evaluated on a same dataset with different calibration pairs. The passing rate of an IMRT QA planar dose was calculated by using 3 mm and 3% criteria and compared with respect to each calibration pairs. Results: In this study, a dose variation up to 0.67% for matched photons and 1.0% for electron beams is observed. Differences of flatness and symmetry can be as high as 1% and 0.7% respectively. Gamma analysis shows a passing rate ranging from 34% to 85% for the standard 10 × 10 cm field. Conclusion: Our work demonstrated that a customized array calibration and dose calibration for each machine is preferred to fulfill a high standard patient QA task.},
doi = {10.1118/1.4955756},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • A comparison of the AAPM Protocol for the determination of absorbed dose from high-energy photon and electron beams (TG21) with currently used protocols for electron and photon dosimetry is presented. These protocols are the International Commission on Radiation Units and Measurements Report 21, Radiation Dosimetry: Electrons with Initial Energies Between 1 and 50 MeV (ICRU21), and the AAPM Protocol for the Dosimetry of X- and Gamma Ray Beams with Maximum Energies Between 0.6 and 50 MeV (SCRAD). Assuming a given radiation exposure and chamber parameters, doses to water at dmax for electron beams and at 5 g/cm2 for photon beamsmore » are calculated using the three protocols and then compared. The doses for photon beams calculated using the TG21 and SCRAD protocols are found to differ by 3% or less at energies below 10 MeV. The largest differences occur in photon doses at high energies where the dose calculated with the TG21 protocol is as much as 5.5% greater than that calculated with the SCRAD protocol for a typical thimble ionization chamber. For low electron beam energies, the doses calculated with the ICRU21 protocol are as much as 5% less than TG21 doses when using thimble chambers constructed of tissue-equivalent materials in a water phantom. If dosimetry measurements are performed in polystyrene, the dose calculated using TG21 may be greater than the ICRU21 dose, depending on chamber size and composition. An explanation for some of the differences between the protocols is presented emphasizing the dependence on chamber geometry, chamber composition, and phantom composition.« less
  • Dose measurements using Fricke and ionization methods were compared for {sup 60} Co gamma rays, 4--25-MV photons, and 10--25-MeV electrons. Fricke derived doses based on a constant yield ({epsilon}{sub {ital m}}{ital G}) were in good agreement with ionization derived doses based on the American Association of Physicists in Medicine Task Group 21 protocol and the National Research Council of Canada (NRC) {ital N}{sub {ital D}} calibration or the NRC proposed {ital N}{sub {ital x}}. These measurements also confirmed the validity of the double-voltage technique in the collection efficiency correction, even for swept electron beams. Assuming the correctness of the ionizationmore » derived doses, the radiation yield appeared to be 1% higher and to increase with photon energy when irradiation vessels were made of Pyrex but not with polystyrene cells. These glass wall effects could be due to the scattering perturbation of electrons between inhomogeneous materials and, in particular for photon beams, due to the mismatch in mass energy absorption ratios and mass collision stopping power ratios between the Fricke dosimeter and the wall materials.« less
  • Purpose: The dose in the buildup region of a photon beam is usually determined by the transport of the primary secondary electrons and the contaminating electrons from accelerator head. This can be quantified by the electron disequilibrium factor, E, defined as the ratio between total dose and equilibrium dose (proportional to total kerma), E = 1 in regions beyond buildup region. Ecan be different among accelerators of different models and/or manufactures of the same machine. This study compares E in photon beams from different machine models/ Methods: Photon beam data such as fractional depth dose curve (FDD) and phantom scattermore » factors as a function of field size and phantom depth were measured for different Linac machines. E was extrapolated from these fractional depth dose data while taking into account inverse-square law. The ranges of secondary electron were chosen as 3 and 6 cm for 6 and 15 MV photon beams, respectively. The field sizes range from 2x2 to 40x40 cm{sup 2}. Results: The comparison indicates the standard deviations of electron contamination among different machines are about 2.4 - 3.3% at 5 mm depth for 6 MV and 1.2 - 3.9% at 1 cm depth for 15 MV for the same field size. The corresponding maximum deviations are 3.0 - 4.6% and 2 - 4% for 6 and 15 MV, respectively. Both standard and maximum deviations are independent of field sizes in the buildup region for 6 MV photons, and slightly decreasing with increasing field size at depths up to 1 cm for 15 MV photons. Conclusion: The deviations of electron disequilibrium factor for all studied Linacs are less than 3% beyond the depth of 0.5 cm for the photon beams for the full range of field sizes (2-40 cm) so long as they are from the same manufacturer.« less
  • Purpose: This study investigates the calibration error of detector sensitivity for MapCheck due to inaccurate positioning of the device, which is not taken into account by the current commercial iterative calibration algorithm. We hypothesize the calibration is more vulnerable to the positioning error for the flatten filter free (FFF) beams than the conventional flatten filter flattened beams. Methods: MapCheck2 was calibrated with 10MV conventional and FFF beams, with careful alignment and with 1cm positioning error during calibration, respectively. Open fields of 37cmx37cm were delivered to gauge the impact of resultant calibration errors. The local calibration error was modeled as amore » detector independent multiplication factor, with which propagation error was estimated with positioning error from 1mm to 1cm. The calibrated sensitivities, without positioning error, were compared between the conventional and FFF beams to evaluate the dependence on the beam type. Results: The 1cm positioning error leads to 0.39% and 5.24% local calibration error in the conventional and FFF beams respectively. After propagating to the edges of MapCheck, the calibration errors become 6.5% and 57.7%, respectively. The propagation error increases almost linearly with respect to the positioning error. The difference of sensitivities between the conventional and FFF beams was small (0.11 ± 0.49%). Conclusion: The results demonstrate that the positioning error is not handled by the current commercial calibration algorithm of MapCheck. Particularly, the calibration errors for the FFF beams are ~9 times greater than those for the conventional beams with identical positioning error, and a small 1mm positioning error might lead to up to 8% calibration error. Since the sensitivities are only slightly dependent of the beam type and the conventional beam is less affected by the positioning error, it is advisable to cross-check the sensitivities between the conventional and FFF beams to detect potential calibration errors due to inaccurate positioning. This work was partially supported by a DOD Grant No.; DOD W81XWH1010862.« less
  • Purpose: To compare the Varian aS-1000 EPID imager to the isocentrically mounted MapCHECK 2 diode array for RapidArc QAs as a function of photon beam energy. Methods: A Varian TrueBeam STx with an aS-1000 digital imaging panel was used to acquire RapidArc QA images for 13 patient plans; each plan QA was performed at 6, 8, 10 and 15MV energies. The Portal Dose Image Prediction algorithm in the Varian Eclipse treatment planning system (TPS) was used to create the comparison image for the EPID acquisition. A Sun Nuclear MapCHECK 2 diode array on an isocentric mounting fixture with 5 cmmore » water-equivalent buildup was also used for the RapidArc QAs. A composite dose plane was taken from the Eclipse TPS for comparison to the MapCHECK 2 measurements. A gamma test was implemented in the Sun Nuclear Patient software with 10% threshold and absolute comparison for both QA methods. The two-tailed paired t-test was employed to analyze the statistical significance between two methods at the 95% confidence level. Results: The average gamma passing rates were greater than 95% at 3%/3mm using both methods for all four energies. The average passing rates were within 2.5% and 1.1% of each other when analyzed at 2%/2mm and 3%/3mm conditions, respectively. The EPID passing rates were somewhat better than the MapCHECK 2 when analyzed at 1%/1mm condition; this difference decreased with increasing energy (9.1% at 6MV to 2.7% at 15MV). The differences were not statistically significant for all criteria and energies (p-value ã 0.05). Conclusion: EPID-based RapidArc QA results are comparable to MapCHECK 2 when using 3%/3mm criteria at all four energies. EPID-based QA shows potential for being the superior device under strict gamma criteria.« less