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Original Article

Korean Journal of Medical Physics 2012; 23(2): 81-90

Published online June 25, 2012

Copyright © Korean Society of Medical Physics.

Evaluation of Setup Uncertainty on the CTV Dose and Setup Margin Using Monte Carlo Simulation

몬테칼로 전산모사를 이용한 셋업오차가 임상표적체적에 전달되는 선량과 셋업마진에 대하여 미치는 영향 평가

Ilsung Cho, Jungwon Kwark, Byungchul Cho, Jong Hoon Kim, Seung Do Ahn, Sung Ho Park

조일성ㆍ곽정원ㆍ조병철ㆍ김종훈ㆍ안승도ㆍ박성호

Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea

울산대학교 의과대학 서울아산병원 방사선종양학교실

Abstract

The effect of setup uncertainties on CTV dose and the correlation between setup uncertainties and setup margin were evaluated by Monte Carlo based numerical simulation. Patient specific information of IMRT treatment plan for rectal cancer designed on the VARIAN Eclipse planning system was utilized for the Monte Carlo simulation program including the planned dose distribution and tumor volume information of a rectal cancer patient. The simulation program was developed for the purpose of the study on Linux environment using open source packages, GNU C++ and ROOT data analysis framework. All misalignments of patient setup were assumed to follow the central limit theorem. Thus systematic and random errors were generated according to the gaussian statistics with a given standard deviation as simulation input parameter. After the setup error simulations, the change of dose in CTV volume was analyzed with the simulation result. In order to verify the conventional margin recipe, the correlation between setup error and setup margin was compared with the margin formula developed on three dimensional conformal radiation therapy. The simulation was performed total 2,000 times for each simulation input of systematic and random errors independently. The size of standard deviation for generating patient setup errors was changed from 1 mm to 10 mm with 1 mm step. In case for the systematic error the minimum dose on CTV

D _{min} ^{stat.} `
was decreased from 100.4 to 72.50% and the mean dose
bar{D} ` _{syst.}
was decreased from 100.45% to 97.88%. However the standard deviation of dose distribution in CTV volume was increased from 0.02% to 3.33%. The effect of random error gave the same result of a reduction of mean and minimum dose to CTV volume. It was found that the minimum dose on CTV volume
D _{min} ^{rand.}
was reduced from 100.45% to 94.80% and the mean dose to CTV
bar{D} ` _{rand.}
was decreased from 100.46% to 97.87%. Like systematic error, the standard deviation of CTV dose
DELTA D _{rand}
was increased from 0.01% to 0.63%. After calculating a size of margin for each systematic and random error the "population ratio" was introduced and applied to verify margin recipe. It was found that the conventional margin formula satisfy margin object on IMRT treatment for rectal cancer. It is considered that the developed Monte-carlo based simulation program might be useful to study for patient setup error and dose coverage in CTV volume due to variations of margin size and setup error.

KeywordsIMRT, Setup uncertainty, Setup margin, Monte Carlo simulation, Treatment planning

Korean Society of Medical Physics

Vol.35 No.2
June 2024

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

Frequency: Quarterly

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