Planning and Dosimetric Study of Volumetric Modulated Arc Based Hypofractionated Stereotactic Radiotherapy for Acoustic Schwannoma - 6MV Flattening Filter Free Photon Beam

Background: The purpose of this study was to assess the dosimetric and clinical feasibility of volumetricmodulated arc based hypofractionated stereotactic radiotherapy (RapidArc) treatment for large acousticschwannoma (AS >10cc). Materials and
Methods: Ten AS patients were immobilized using BrainLab mask.They were subject to multimodality imaging (magnetic resonance and computed tomography) to contour targetand organs at risk (brainstem and cochlea). Volumetric modulated arc therapy (VMAT) based stereotactic planswere optimized in Eclipse (V11) treatment planning system (TPS) using progressive resolution optimizer-III andfinal dose calculations were performed using analytical anisotropic algorithm with 1.5 mm grid resolution. All ASpresented in this study were treated with VMAT based HSRT to a total dose of 25Gy in 5 fractions (5fractions/week). VMAT plan contains 2-4 non-coplanar arcs. Treatment planning was performed to achieve at least 99%of PTV volume (D99) receives 100% of prescription dose (25Gy), while dose to OAR’s were kept below thetolerance limits. Dose–volume histograms (DVH) were analyzed to assess plan quality. Treatments were deliveredusing upgraded 6 MV un-flattened photon beam (FFF) from Clinac-iX machine. Extensive pretreatment qualityassurance measurements were carried out to report on quality of delivery. Point dosimetry was performed usingthree different detectors, which includes CC13 ion-chamber, Exradin A14 ion-chamber and Exradin W1 plasticscintillator detector (PSD) which have measuring volume of 0.13 cm3, 0.009 cm3 and 0.002 cm3 respectively.
Results: Average PTV volume of AS was 11.3cc (±4.8), and located in eloquent areas. VMAT plans providedcomplete PTV coverage with average conformity index of 1.06 (±0.05). OAR’s dose were kept below tolerancelimit recommend by American Association of Physicist in Medicine task group-101(brainstem V0.5cc < 23Gy,cochlea maximum < 25Gy and Optic pathway <25Gy). PSD resulted in superior dosimetric accuracy comparedwith other two detectors (p=0.021 for PSD