Comparison of a micro-multileaf collimator with a 5-mm-leaf-width collimator for intracranial stereotactic radiotherapy

Abstract

Purpose To dosimetrically compare a micro-multileaf collimator (minimum leaf width of 3 mm) with the 5-mm-leaf multileaf collimator (MLC) of a standard linear accelerator for stereotactic conformal radiotherapy treatment of intracranial lesions. Methods and materials Fourteen patients previously treated for a variety of irregularly shaped intracranial lesions using BrainLAB's micro-MLC were retrospectively replanned using the Varian Millennium MLC (5 mm leaf width). All planning was performed with the BrainSCAN v 5.1 software. The same fixed, noncoplanar beam arrangement was used for both plans, and identical target coverage was achieved by adjusting the MLC shape around the planning target volume (PTV). The isodose distributions and dose–volume histograms (DVH) were computed and plans were compared in terms of conformity of the prescription isodose to the PTV and dose received by surrounding normal tissue. Results Equivalent PTV coverage was achieved using the 5-mm collimator by adjusting the MLC shape around the target in every case. There was a statistically significant increase in the conformity index for the Varian MLC compared with the micro-MLC ( p < 0.001), indicating a worse conformity of the prescription isodose to the PTV, but this parameter was within our (and Radiation Therapy Oncology Group) clinical criterion in all cases. There was no statistically significant difference in the maximum dose to critical structures, but DVH curves demonstrated an increased volume of normal tissue irradiated to the lower isodose levels. The mean increase in the volume of critical structure enclosed within the 50% and 70% isodose surfaces was 5.7% and 4.9%, respectively. Conclusions The micro-MLC consistently improves both PTV conformity and surrounding tissue sparing when compared to that of a standard linear accelerator. However, when viewed quantitatively, the improvements are small enough that individual centers may question their choice of equipment when outfitting a stereotactic radiotherapy service.