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KMID : 0384620120230040317
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Korean Journal of Medical Physics
2012 Volume.23 No. 4 p.317 ~ p.325
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Application of Off-axis Correction Method for EPID Based IMRT QA
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Cho Il-Sung
Kwak Jung-Won
Park Sung-Ho
Ahn Seung-Do
Jeong Dong-Hyeok
Cho Byung-Chul
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Abstract
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The Varian PORTALVISION (Varian Medical Systems, US) shows significant overresponses as the off-center distance increases compared to the predicted dose. In order to correct the dose discrepancy, the off-axis correction is applied to VARIAN iX linear accelerators. The portal dose for 38×28 cm2 open field is acquired for 6 MV, 15 MV photon beams and also are predicted by PDIP algorithm under the same condition of the portal dose acquisition. The off-axis correction is applied by modifying the 40×40 cm2 diagonal beam profile data which is used for the beam profile calibration. The ratios between predicted dose and measured dose is modeled as a function of off-axis distance with the 4th polynomial and is applied to the 40×40 cm2 diagonal beam profile data as the weight to correct measured dose by EPID detector. The discrepancy between measured dose and predicted dose is reduced from 4.17±2.76 CU to 0.18±0.8 CU for 6 MV photon beam and from 3.23±2.59 CU to 0.04±0.85 CU for 15 MV photon beam. The passing rate of gamma analysis for the pyramid fluence patten with the 4%, 4 mm criteria is improved from 98.7% to 99.1% for 6 MV photon beam, from 99.8% to 99.9% for 15 MV photon beam. IMRT QA is also performed for randomly selected Head and Neck and Prostate IMRT plans after applying the off-axis correction. The gamma passing rare is improved by 3% on average, for Head and Neck cases: 94.7±3.2% to 98.2±1.4%, for Prostate cases: 95.5±2.6%, 98.4±1.8%. The gamma analysis criteria is 3%, 3 mm with 10% threshold. It is considered that the off-axis correction might be an effective and easily adaptable means for correcting the discrepancy between measured dose and predicted dose for IMRT QA using EPID in clinic.
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KEYWORD
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IMRT, Quality assurance, EPID, Off-axis correction algorithm
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