Tumor volume threshold for achieving improved conformity in VMAT and Gamma Knife stereotactic radiosurgery for vestibular schwannoma

Abstract

Background and purpose: Recent advances in multileaf collimator field shaping technology and inverse planning software have resulted in highly conformal LINAC based stereotactic radiosurgery (SRS) plans with minimal dose to critical structures. This modeling study compares Gamma Knife (GK) and LINAC SRS for vestibular schwannoma (VS).

Materials and methods: 76 treatment plans from nineteen patients with VS were planned using GK forward planning and volumetric arc therapy (VMAT) inverse planning software. VMAT plans were generated with 1 coplanar, 3 and 5 non-coplanar arcs. Dose to normal structures and beam-on time (dose rate 600MU/min) were compared using Kruskal-Wallis and Dunn's post hoc test.

Results: Median tumor volume was 1.2cm(3) (range 0.1-4.8cm(3)). A peripheral tumor dose of 12Gy was prescribed. Tumor coverage was >99.8%. VMAT plans had lower target D2% and mean dose, as well as decreased beam-on time, compared to GK plans (p<0.0001). Paddick conformity index in VMAT 5 arc plans was superior to that of GK plans for targets >0.5cm(3) (p=0.002). Similar dose to cochlea, normal brain tissue and brainstem was observed.

Conclusion: VMAT should be considered as a safe, alternative modality to GK for VS SRS treatment, especially for tumors larger than 0.5cm(3).

Keywords: Gamma knife; Radiosurgery; VMAT; Vestibular schwannoma.