SU‐E‐T‐220: Comparison of Two Different Setups for VMAT Patient‐Specific QA

Abstract

Purpose: Patient‐specific QA methods are needed for verification of intensisty‐modulated radiation therapy (IMRT). VMAT treatment is delivered through dynamic arcs with a continuous variation of gantry speed, dose rate and MLC position. 2D ion chamber arrays can be used to compare dose distributions as a part of patient‐specific QA procedure in VMAT technique. This work shows the gamma tests results for two different setups using a 2D array in the verification of VMAT plans. Methods: VMAT treatments are delivered with an Elekta Synergy linac. A 2D ion chamber array (MatriXX Evolution, IBA Dosimetry) is used to measure and evaluate dose distributions calculated by the TPS (Monaco 3.1, Elekta). Detector is calibrated to perform absolute dose measurements. Two different setups are presented as a part of QA verification procedure. First setup mounts the MatriXX detector inserted in a dedicated gantry holder with SSD of 100 cm. Dose measurements are compared with calculations collapsed to 0° because the detector is attached to the gantry during the gantry arcing. Second setup consists on MatriXX inserted in a cubic and homogeneous phantom (MultiCube, IBA Dosimetry). Coronal and sagital dose planes are measured. A set of correction factors are needed in order to correct dose in this arrangement. The sampling time is 150 ms. 111 patients have been verified with previous setups with a gamma index criteria of 3%/3mm. Results: Mean value for % points meeting gamma test criteria is (99.02±1.14)% in the first setup. The worst value is 92.9%. Results for second setup are (99.05±0.67)% for the coronal planes and (98.19±1.10)% for the sagital planes. The worst value is 95.22% for a sagital plane. Conclusion: The suitability of MatriXX detector for patient‐specific QA in VMAT techniques has been tested for two different setups with excellent agreement between calculation and measurement.