[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.