Effects of multileaf collimator parameters on treatment planning of intensity-modulated radiotherapy

In inverse planning of intensity-modulated radiotherapy (IMRT), the setting of multileaf collimator (MLC) parameters affects the optimization algorithms and dose distribution. We investigated the effects of varying the MLC leaf width, leaf insertion percentage, and leaf increment in treatment planning of IMRT in 3 cancer cases: nasopharynx, esophagus, and prostate. Inverse planning of the 3 cancer cases was performed using the XiO treatment planning system. MLCs with 0.5 and 1.0 cm were used to evaluate the leaf width effect, whereas leaf insertions of 20%, 50%, and 80% were used to demonstrate the effect of leaf insertion percentage, and leaf increments of 0.5, 1.0, and 2.0 cm were used to study the leaf increment effect. The treatment plans were evaluated by dose profiles, tumor control probability (TCP), and normal tissue complication probability (NTCP). The 0.5-cm MLC leaves showed better TCPs and NTCPs than the 1.0-cm leaves in the 3 cancer cases, although the differences were less than 2.5%. For the leaf insertion percentage, the dose profile differences among the 3 levels of increments were minimal, and their differences in TCP and NTCP were extremely small (< 1.5%). The effect of leaf increment was more prominent, dose profile, TCPs, and NTCPs were best for the smallest leaf increment and they deteriorated as the leaf increment increased. Narrower leaves gave slightly better sparing of organs at risk (OAR)s; changing the leaf insertion percentage brought about negligible changes, whereas increasing the leaf increment significantly degraded the treatment plans.