Introduction: Thirteen patients diagnosed with vestibular schwannoma were treated with radiosurgery between June 2011 and February 2013 using the Novalis Tx linear accelerator incorporating Varian HD120 multileaf collimator system (MLC). The HD120 MLC has leaf widths of 2.5mm, which offer finer resolution over the previous designs: Millenium (5mm) and m3 (3mm). Differences in target conformality and sparing of normal tissue were investigated using each MLC design.
Methods: Gross tumor volumes were defined by a neurosurgeon and radiation oncologist. In-vitro measurements of the dose characteristics of each MLC were evaluated by a medical physicist. The dosimetric performance of each MLC was compared by creating radiosurgery plans for each patient using each MLC. The resulting plans were evaluated for conformity to the target volume and dose to surrounding normal tissue.
Results: All patients were prescribed 12Gy to 95% of the target volume. The mean conformity index (Paddick Index) was 2.23 (45.3%), 2.10(47.3%) and 2.08(49.1%) for Millenium, m3 and HD120 MLC respectively. The maximum dose to the brainstem was 13.20, 13.17 and 13.07Gy for Millenium, m3 and HD120 MLC respectively. For dynamic conformal arc plans (n=8) the HD120 produced a lower maximum dose to the brainstem for target volumes less than 1.0cc. The mean brainstem dose was lowest for the HD120 (0.63Gy) and highest for the Millenium (0.70Gy). Optic apparatus received less than 1Gy from all plans. In phantom measurements of the MLCs demonstrated reduced transmission from 2.5% for m3 to 1.3% for HD120.
Conclusions: Developments in the design of the MLC for radiosurgery have enabled improved conformity for the target dose while reducing dose to normal tissue. The advantages are more significant for small volume targets such as vestibular schwannomas. Radiographic followup was available for seven patients which indicated no progression of disease and decreased enhancement on post contrast MR.
Patient Care: This work identifies a subset of vestibular schwannoma patients that would benefit most from high resolution multileaf collimator radiosurgery.
Learning Objectives: Following this session participants will be able to:
1. Understand how different collimator systems used in radiosurgery affect plan quality.
2. Discuss advantages of different multileaf collimators for radiosurgery in a multidisciplinary setting.
3. Identify lesions that would benefit most from fine resolution multileaf collimators.