Evaluation of two detector arrays for VMAT pre-treatment quality assurance

Abstract

Introduction Patient-specific pre-treatment quality assurance (QA) is a necessary prerequisite to VMAT treatment. 2D detector arrays have become the standard device for verification of intensity modulated dose distributions. Purpose Aim of this work is to verify the suitability of two ionization chamber 2D arrays (PTW Octavius4D 1500 and PTW Octavius4D 729) for pre-treatment VMAT plan verification and to compare the results achieved by the two systems. Materials and methods Measurements were performed by Octavius4D 1500 and Octavius4D 729 in the Octavius4D phantom. Forty VMAT plans elaborated by TPS Elekta Monaco ® 5.0 and delivered by Elekta Synergy ® Linac (6, 10 MV) have been used in this study (10 head and neck, 10 breast, 10 prostate and 10 pelvic treatments). 3D gamma metric at 3% L/3 mm has been used to compare measured and computed maps. Results Mean gamma pass rate for the Octavius4D 1500 was 93.9% (s.d. 3.5%) for HN plans, 95.6% (s.d. 2.4%) for breast treatments, 98.9% (s.d. 1.7%) for prostate and 95.6% (s.d. 2.2%) for pelvic plans. Mean gamma pass rate for the Octavius4D 729 was 92.4% (s.d. 4.9%) for HN plans, 80.7% (s.d. 11.9%) for breast treatments, 97.8% (s.d. 5.1%) for prostate and 87.6% (s.d. 4.3%) for pelvic plans. Improve measurement resolution by merging two longitudinally shifted measurements resulted in a better agreement between measured and computed maps for both dosimetric devices. Conclusion Both investigated systems resulted suitable for VMAT pre-treatment QA, but Octavius 4D 1500 outperformed its predecessor Octavius4D 729 because of the higher spatial resolution of the dosimeters.