The Effect of MLC Leaf Width in Single-Isocenter Multi-target Radiosurgery with Volumetric Modulated Arc Therapy

Abstract

Purpose

Single-isocenter multi-target (SIMT) Volumetric Modulated Arc Therapy (VMAT) technique can produce highly conformal dose distributions and short treatment delivery times for the treatment of multiple brain metastases. SIMT radiosurgery using VMAT is primarily limited to linear accelerators utilizing 2.5mm leaf width MLCs. We explore feasibility of applying this technique to linear accelerators utilizing MLCs with leaf width of 5mm to broaden the applicability of SIMT radiosurgery using VMAT to include the greater number of linear accelerators with standard 5mm MLCs.

Methods

Twenty patients with 3-10 intracranial brain metastases originally treated with 2.5 mm leaf width MLCs were re-planned using standard 5mm leaf width MLCs and the same treatment geometry (3-5 VMAT arcs). Conformity index, low (V30%), and moderate (V50%) isodose spill were selected for analysis. V12Gy was also analyzed for single fraction cases. We tested the effects of several strategies to mitigate degradations of dose quality values when 5 mm leaf width MLCs were used; these included duplicating each VMAT arc with altered collimator angles by 10°, 15°, and 90°, and adding 1-2 VMAT arcs, with all arcs equally spaced.

Results

Wider MLCs caused small changes in total MUs (5827±2334 vs 5572±2220, p=.006), and Conformity Index (CI) (2.22%±0.05%, p=.045), but produced more substantial increases in brain V30%[%] and V50%[%] (27.75%±0.16% and 20.04%±0.13% respectively, p < .001 for both), and V12Gy[cc] (16.91%±0.12%, p < .001).

Conclusion

SIMT radiosurgery delivered via VMAT using 5mm leaf width MLCs can achieve similar CI compared to that using 2.5mm leaf width MLCs but with moderately increased isodose spill, which can be only partially mitigated by increasing the number of VMAT arcs.