自作プログラムを用いた照射中のマルチリ一フコリメ 一夕動作の評価

摘要

強度変調放射線治療(intensity modulated radiation therapy: IMRT)は，空間的，時間的に不均一な放射線強度をもつ照射ビームを多方向から照射することによつて，病巣部に最適な線量分布を得る放射線治療法のことである。 IMRTでは，マルチリ一フコリメータ(multi-leaf collimator: MLC)を用いた照射法が代表的で，step and shoot IMRT (segmental MLC: SMLC方式）とsliding window IMRT (dynamic MLC: DMLC方式)に分類される。%Multi-leaf collimators (MLCs) are used to modulate intensity during intensity modulated radiation therapy (IMRT). Evaluation of MLC movement in IMRT is important, since the accuracy of MLC movements affects the dose distribution. This evaluation is conventionally performed using an attached Dynalog File Viewer (DFV). However, due to its being an overall evaluation, it is not possible to discover significant errors. In this study, we developed software that permits easy analysis of MLC movements that can be used to retrospectively evaluate MLC movement during irradiation. We also evaluated the usefulness of our in-house program and confirmed its potential for use in clinical scenarios. We created a program that can read MLC logfiles using Visual Basic 6.0 and visualize the temporal changes and movements of the MLC. To evaluate our in-house program's efficacy in analyzing dynamic MLC-QA (quality assurance), we compared the numerical results yielded by our in-house program and the DFV. The results showed that our in-house program was able to reveal errors below the error root mean square (RMS) values obtained using the DFV. Using irregular surface compensator (ISC) irradiation conditions in a clinical context, we compared our in-house program with the DFV and, using RMS analysis, identified cases that showed excessive error. Our in-house program can also be used to investigate whether unacceptable errors are present, as well as their cause, when using the MLC, as it allows easy real-time observation and evaluation of MLC movements. An additional benefit is that collecting the MLC lognle during actual treatment also allows it to be evaluated retrospectively after continuous MLC operation.