SU‐E‐T‐799: Verification of a Simultaneous Treatment of Multiple Brain Metastases Using VMAT Technique by a Composite Alanine‐Gel Dosimeter Phantom

Abstract

Purpose:This work presents an end‐to‐end test using a Gel‐Alanine phantom to validate the three‐dimensional (3D) dose distribution (DD) delivered by a single isocenter VMAT technique on the simultaneous treatment of multiple brain metastases.Methods:Three cylindrical phantons containing MAGIC‐f gel dosimeter were used to measure the 3D DD of a VMAT treatment, the first two were filled with the gel dosimeter (Gel 1 and 2) and the third one was filled with gel and 12 alanine dosimeters distributed along it (Gel 3). Gels 1 and 3 were irradiated and gel 2 was used to map the magnetic resonance image (MRI) scanner field inomogeneities. A CT scan of gel 3 was used for the VMAT treatment planning and 5 alanine pellets were chosen as lesions, around them a PTV was grown and different dose prescriptions were assigned for each one, varying from 5 to 9Gy. Before treatment, the plan was approved in a QA based on an ionization chamber absolute dose measurement, a radiochromic film planar dose measurement and a portal dosimetry per field verification; and also the phantons positioning were verified by ExacTrac 6D correction and OBI kV Cone Beam CT. The gels were irradiated, the MRIs were acquired 24 hours after irradiation and finally, the alanine dosimeters were analysed in a X‐band Electron Spin Resonance spectrometer.Results:The association of the two detectors enabled the 3D dose evaluation by gel and punctually inside target volumes by alanine. In the gamma analyses (3%/3mm) comparing the 5 PTVs’ central images DD with TPS expected DD more than 95% of the points were approved. The alanine absolute dose measurements were in agreement with TPS by less than 5%.Conclusion:The gel‐alanine phantom enabled the dosimetric validation of multiple brain metastases treatment using VMAT, being an almost ideal tool for this application.This work is partially supported by FAPESP.