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

Shanmugam Thirumalai Swamy,Murugesan Kathirvel,Sai Subramanian,Gandhi Arun,Chandrasekaran Anu Radha

doi:10.7314/apjcp.2015.16.12.5019

Shanmugam Thirumalai Swamy, Murugesan Kathirvel + Show 3 more

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https://doi.org/10.7314/apjcp.2015.16.12.5019

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Abstract

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 acoustic schwannoma (AS>10 cc). Ten AS patients 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 AS presented in this study were treated with VMAT based HSRT to a total dose of 25 Gy in 5 fractions (5 fractions/ 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 (25 Gy), 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. Average PTV volume of AS was 11.3 cc (±4.8), and located in eloquent areas. VMAT plans provided complete PTV 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 V0.5 cc<23 Gy, cochlea maximum<25 Gy and Optic pathway<25 Gy). PSD resulted in superior dosimetric accuracy compared with other two detectors (p=0.021 for PSD.

Highlights

Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) have been used for several decades in the treatment of benign and malignant lesions as well as functional disorders
For the past 10 years, radiation treatment for vestibular schwannoma has been increasingly used as an alternative to microsurgery because it is claimed that it results in high rates of control and elimination of the operative morbidity and because early outcomes were better for patients having stereotactic radiotherapy compared with surgical resection (Kapoor et al, 2011)
Tumor control rates reported in linear accelerator series, where the stereotactic irradiation has been given in fractionated mode, are on par with the reported outcome in the largest radio-surgery series using one fraction (Gamma Knife)

Summary

Introduction

Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) have been used for several decades in the treatment of benign and malignant lesions as well as functional disorders. The major feature that separates stereotactic treatment from conventional radiation treatment is the delivery of large doses in one or few fractions, which results in a high biological effective dose (BED) (Ingrosso et al, 2012). The SRS (12-14Gy in single fraction) approach to the treatment of small to medium sized AS was well consolidated in literature with results demonstrating both very high local control rates (larger than 90% up to 96-98%) as well as minimal toxicity (5 year radiation related toxicity as low as ~5-10%) (Kappor et al, 2011). 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 acoustic schwannoma (AS >10cc). 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 the tolerance limits. PSD resulted in superior dosimetric accuracy compared with other two detectors

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