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Original Article

Korean Journal of Medical Physics 2012; 23(4): 317-325

Published online December 25, 2012

Copyright © Korean Society of Medical Physics.

Application of Off-axis Correction Method forEPID Based IMRT QA

EPID를 사용한 세기조절방사선치료의 정도관리에 있어축이탈 보정(Off-axis Correction)의 적용

Ilsung Cho*, Jungwon Kwark*, Sung Ho Park*, Seung Do Ahn*, Dong Hyeok Jeong, Byungchul Cho*

조일성*ㆍ곽정원*ㆍ박성호*ㆍ안승도*ㆍ정동혁ㆍ조병철*

*Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Research Center, Dong-nam Institute of Radiological & Medical Sciences, Busan, Korea

*울산대학교 의과대학 서울아산병원 방사선종양학과, 동남권원자력의학원 연구센터

Abstract

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.

KeywordsIMRT, Quality assurance, EPID, Off-axis correction algorithm

Korean Society of Medical Physics

Vol.35 No.1
March 2024

pISSN 2508-4445
eISSN 2508-4453
Formerly ISSN 1226-5829

Frequency: Quarterly

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