Impact of a micromultileaf collimator on stereotactic radiotherapy of uveal melanoma

Abstract

Purpose: To evaluate the impact of a micro multileaf collimator (mMLC) on Linac-based stereotactic radiotherapy (SRT) of uveal melanoma by comparing circular arc with static conformal, dynamic arc, and intensity-modulated SRT. Methods and materials: Forty uveal melanoma patients were selected from ∼100 patients treated with SRT since 1996. For each patient, four treatment plans (BrainSCAN XL, V5.0) were made: conventional arc, static conformal, dynamic arc plan, and intensity-modulated radiotherapy (IMRT). The goal of treatment planning was to fully encompass the planning target volume (PTV) by the 80% isodose while minimizing doses to the optic nerve and lens. The following parameters were evaluated: target conformity; target homogeneity; ratio of the target volume and 50% isodose volume; normal tissue receiving doses ≥80%, ≥50%, and ≥20%; central nervous system volume irradiated to ≥20%; optical nerve volume irradiated ≥50%, D max of the lens; lens volume receiving ≥20%; and monitor units. Results: PTVs ranged from 0.68 to 4.90 cm 3 (mean 1.97 ± 0.97 cm 3). The average reduction of the prescription isodose volume was 1–1.5 cm 3 for conformal (range 2.6–0.3 cm 3), dynamic arc (range 2.5–0.3 cm 3), and IMRT plans (range 3.9–0.1 cm 3), compared with conventional arc therapy. Central nervous system volumes irradiated to doses ≥20% were smallest for conventional or dynamic arc treatments. Average target dose homogeneity values were 1.74 ± 0.50 for arc, 1.27 ± 0.02 for static mMLC, 1.26 ± 0.01 for dynamic arc, and 1.15 ± 0.03 for IMRT plans. IMRT helped to reduce doses to the lens but did not provide an advantage for optical nerve sparing. When applying IMRT, the monitor units increased by approximately one-third compared with static mMLC–based SRT. Conclusion: Conformal mMLC and dynamic arc SRT are the treatment options of choice for Linac-based SRT of uveal melanoma. They present dosimetric advantages, while being highly efficient in treatment planning and delivery.