Dynamic MLC log files are recorded when intensity modulated radiation therapy(IMRT) irradiation is completed. These files are not information about irradiated dose, but only a report of the MLC position at each dose segment. To acquire the error of irradiated dose is important from the quality assurance (QA) point of view. The purpose of this study is to develop a program to estimate the error of irradiated dose by analyzing MLC log files and verify their calculation accuracy. First, we measured doses of sliding slit fields with the leaf gap of 12 types (3 - 100 mm) which set the gap error of 12 types (-1.0 - +2.0 mm) respectively. To obtain a graph that indicates the relationship between the gap error and the dose error, we determined a conversion function from its approximation curve. Second, the leaf gap and the gap error at each dose segment were calculated from analyzing the MLC log files. From these data, the program that calculates the errors of estimating irradiated dose from the MLC log files was created. Finally, to verify the calculation accuracy, we compared the calculated errors with the measured errors using sliding slit fields with the leaf gap of eight types (3 - 100 mm) which set the monitor unit of six types (10 - 200 MU) respectively. The conversion function from the approximation curve showed better agreement to the original curve in the case of the leaf gap of 50 mm or less. In the sliding slit fields measurement, the irradiated dose error between the calculated and the measured were -1.61% to +2.37% in the leaf gap of 50 mm or less, and were -0.26% to +11.35% in the leaf gap of 50 mm or over. Because of an approximation error of the conversion function, there was a trend that the larger the leaf gap, the larger the dose error in the leaf gap of 50 mm or over. Furthermore, in every leaf gap, the lower the MU in which the beam hold-off frequently occurs, the larger the error. Most prostate IMRT plans in our institution have approximately 20 mm of the mean leaf gap and have no influence on low MU because the beam hold-off hardly occurs in these irradiations. Therefore, this program that converts the error of the leaf position to the estimate error of irradiated dose has sufficient accuracy in the leaf gap of 50 mm or less, and is considered to be a useful QA tool for estimating the irradiated dose error for the standard prostate IMRT in our institution.
原文链接
分享
引用
