The Impact of Multileaf Collimator Size on Single Isocenter Dynamic Conformal Arcs-Based Radiosurgery for Brain...

To compare the plan quality of stereotactic radiosurgery (SRS) between 2.5-mm and 5-mm multileaf collimator (MLC) and investigate the factors' influence on the differences by MLC size.

Seventy-six treatment plans including 145 targets calculated with a single isocenter multiple noncoplanar dynamic conformal arc (DCA) technique using automatic multiple brain metastases (MBM) treatment planning system. Conformity index (CI), gradient index (GI), lesion underdosage volume factor (LUF), healthy tissue overdose volume factor (HTOF), geometric conformity index (g), and mean dose to normal organs were compared between 2.5-mm and 5-mm MLC. Then the factors that influenced the differences of these parameters were investigated. The impact of target size was also investigated for CI and GI values of individual targets (n=145), and differences between 2.5-mm and 5-mm MLC were analyzed.

All parameters except for LUF were significantly better in plans with 2.5 mm MLC. Target size was a significant factor for difference in HTOF, and distance between targets was a significant factor for difference in brain dose and GI. Among 145 metastases, the average inverse CI was 1.35 and 1.47 with 2.5-mm and 5-mm MLC, respectively (p<0.001). The average GI was 3.21 and 3.53, respectively (p<0.001). For individual targets, target size was a significant factor in CI and GI both with 2.5-mm and 5-mm MLC (p-value: <0.001, each). CI and GI were significantly better with 2.5-mm than 5-mm MLC. CI was almost >0.67 except for ≤5mm targets with 5-mm MLC. Also, GI was almost smaller than 3.0 for >10 mm targets both with 2.5-mm and 5-mm MLC.

MBM with 5-mm MLC was almost fine. However, it may be better to use a conservative margin for larger metastases. It may also be better to avoid SRS with 5-mm MLC for patients with ≤5 mm target size.