Purpose: To analyze the impact of multileaf collimator (MLC) leaf width in multiple metastases radiosurgery (SRS) considering the target distance to isocenter and rotational displacements.
Methods: 10 plans were optimized. The plans were created with Elements Multiple Mets SRS v2.0 (Brainlab AG, Munchen, Germany). The mean number of metastases per plan was 5 ± 2 [3,9] and the mean volume of GTV was 1.1 ± 1.3 cc [0.02, 5.1]. PTV margin criterion was based on GTV-isocenter distance and target dimensions. Plans were performed using 6 MV with high-definition MLC (HDMLC) and recalculated using a generic 5-mm MLC leaf width. The maximum displacement due to rotational combinations (roll, pitch, yaw) was optimized by a genetic algorithm for HDMLC and 5-mm MLC plans. The original plans were re-calculated using the PTV optimized margin and new dosimetric variations were obtained. The Paddick conformity index (PCI), gradient index (GI) and dose variations in the healthy brain were analyzed.
Results: HDMLC plans had better conformity and higher dose falloff than 5-mm MLC. In all cases, dosimetric parameter differences are statistically significant (p<0.05). The smaller lesion volume, the higher dosimetric differences between both plans. The effect of rotational displacements produced for each target in SRS are not dependent on the MLC leaf width (p>0.05).
Conclusion: The finer HDMLC offers dosimetric advantages compared with the 5-mm MLC leaf width in terms of target conformity and dose to the surrounding organs at risk. However, rotational displacements do not depend on MLC leaf width.