[Effects of Low MU in Respiratory Gated IMRT on MLC Position Accuracy and Dose Distribution].

PURPOSE: The purpose of this research is to clarify the effects of low monitor unit (MU) on multileaf collimator (MLC) position accuracy and dose distribution in intensity modulated radiotherapy (IMRT) using respiratory gated. METHOD: In the phantom experiment, irradiation without respiratory gated and respiratory gated with low MU (3, 5, and 7 MU) were performed, and positional accuracy and dose distribution of MLC were analyzed. MLC positional accuracy was calculated from the log-files and the MLC position error, gap size error, MLC leaf speed were calculated and compared with the planned value. Gamma analysis of the dose distribution obtained from the irradiated films and the dose distribution of the treatment plans were carried out. RESULTS: Without respiratory gated and respiratory gated, the frequency of gap size error that did not exceed 0.2 mm were more than 93% under all conditions. MLC position error increased with increasing MLC leaf speed. The determination coefficient of respiratory gated irradiation was lower by about 20% compared with that without respiratory gated, and variation from the approximate straight line occurs. The output difference due to low MU irradiation during respiratory gated was within 1% of the planned value. Although, the pass rate of gamma analysis differed in tumor size, the dose distribution well conformity at 96% or more for both without respiratory gated and respiratory gated. However, in the comparison of the profile in the MLC movement direction, respiratory gated irradiation at 3 MU showed a difference of about 9% at the edge of the irradiated field and about 6% at the point where the dose rapidly changed. CONCLUSION: It was shown that MLC position accuracy due to stop and go of MLC leaf can be secured even with low MU irradiation of about 3 MU. However, attention should be paid to the dose of risk organs adjacent to the tumor margin.

[Multileaf Collimator Position Accuracy of Respiratory Gated VMAT].

Respiratory gated VMAT (volumetric modulated arc therapy) repeats rapid stop and go operations of a MLC (multileaf collimator) by turning the beam on and off by respiratory gating. The rapid stop and go operations of the MLC during respiratory gated irradiation may induce position error of the MLC and may affect output error and dose distribution. The purpose of this study was to clarify the relationship between the MLC position accuracy of the respiratory gated VMAT and the VMAT parameters. In the method, 1 arc, 2 arcs, and 4 arcs plan were created for the virtual target and irradiation was performed without the gated respiration and with the gated respiration. The respiratory gated system used a RPM (real-time position management system). The MLC position error, gap size error, and the MLC leaf speed were calculated from a log-file. In the histogram of the gap size error, the frequency of falling within the error range up to 0.2 mm was about 12 percentage points higher for the gated respiration. The MLC position error increased with increasing the MLC leaf speed. The correlation coefficient between the MLC leaf speed and the MLC position error exceeded 0.96, showing a strong correlation. Dose rate of VMAT parameters decreased with increasing arc number with the gated respiration and without the gated respiration. Gated irradiation was temporarily stopped, and it decreased by about 27% with respect to the dose rate without the gated respiration. The gantry rotation speed repeated the stop and re-rotation operations when gated irradiation was performed. For all arcs, the rotation speed decreased by about 30% compared with the rotation speed without the gated respiration. The pass rate of gamma analysis for each arc plan was about 95%. No effect on gated irradiation dose distribution was observed. Respiratory gated irradiation reduced dose rate change and gantry rotation speed of the VMAT. Reduction of the MLC leaf speed occurred, and the MLC position error and gap size error decreased. The MLC positional accuracy was secured, and it was confirmed that there was no effect on dose distribution by the respiratory gated VMAT.