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Volume staging for arteriovenous malformation treatment using volumetric modulated arc therapy Mendez, Claudia


Volume staged Stereotactic Radiosurgery (VS-SRS) is an SRS technique for the treatment of large Arteriovenous Malformations (AVMs) in the brain. In VS-SRS, the AVM is divided into stages and each stage is treated in a separate Stereotactic Radiosurgery (SRS) sessions in order to limit the radiation exposure to the normal brain surrounding the AVM. The objective of this study is to investigate the feasibility of VS-SRS for AVMs on a linear accelerator using Volumetric Modulated Arc Therapy (VMAT) and a frameless patient positioning system. A VS-SRS planning technique was developed and end-to-end tests were performed to validate the treatment plans. Dosimetric uncertainties that could result from potential patient setup errors between stage treatments were investigated. Ten patients previously treated with fractionated SRS (fSRS) were selected. The planning target sizes ranged from 1.9 to 4.4 cm in diameter and had volumes from 3.7 to 45.8 cc. The VS VMAT treatment planning technique was developed after analyzing a sample Gamma Knife VS plan, and it maximized the dose to the target, limited the dose at the junction between the stage volumes, and minimized the doses to the normal brain. Equivalent Total Dose in 2 Gy fractions (EQD2) plans were computed to compare the VS treatments to the original fSRS plans. The uncertainties in the dose delivered by the linear accelerator were measured using a microdiamond and GaFchromic EBT3 film. Most VMAT VS plans met the normal tissue constraints and target coverage, depending on their shape, size and location. When compared to fSRS plans, normal brain 12-Gy volume per stage was lower by an average of 14%, average conformity was unchanged and normal tissue maximum dose was higher. EQD2 doses to normal tissues showed that fSRS results in lower doses to brain. Agreement of point dose measurements to treatment planning calculated dose averaged -0.6% and the film measurement showed that setting up the phantom for the second stage did not contribute additional uncertainty to the treatment delivery. In conclusion, this work suggests that VS for AVM treatment can be safely implemented on linear accelerators and provides the groundwork for treatment planning procedures and patient quality control.

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