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

@entity1 @entity5513 @entity343 @entity5427
BACKGROUND: The purpose of this study was to assess the dosimetric and clinical feasibility of volumetric modulated arc based hypofractionated stereotactic radiotherapy (RapidArc) treatment for large @entity5513 ( @entity5427 >10cc). MATERIALS AND METHODS: Ten @entity5427 @entity1 were immobilized using BrainLab mask. They were subject to multimodality imaging (magnetic resonance and computed tomography) to contour target and organs at risk (brainstem and cochlea). Volumetric modulated arc therapy (VMAT) based stereotactic plans were optimized in Eclipse (V11) treatment planning system (TPS) using progressive resolution optimizer-III and final dose calculations were performed using analytical anisotropic algorithm with 1.5 mm grid resolution. All @entity5427 presented 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 @entity343 volume (D99) receives 100% of prescription dose (25Gy), while dose to OAR's were kept below the tolerance limits. Dose-volume histograms (DVH) were analyzed to assess plan quality. Treatments were delivered using upgraded 6 MV un-flattened photon beam (FFF) from Clinac-iX machine. Extensive pretreatment quality assurance measurements were carried out to report on quality of delivery. Point dosimetry was performed using three different detectors, which includes CC13 ion-chamber, Exradin A14 ion-chamber and Exradin W1 plastic scintillator detector (PSD) which have measuring volume of 0.13 cm3, 0.009 cm3 and 0.002 cm3 respectively. RESULTS: Average @entity343 volume of @entity5427 was 11.3cc (  4.8), and located in eloquent areas. VMAT plans provided complete @entity343 coverage with average conformity index of 1.06 (  0.05). OAR's dose were kept below tolerance limit 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 compared with other two detectors (p=0.021 for PSD.
