EPID-based dosimetry using a commercial software: Gamma analysis and DVH metrics for VMAT pre-treatment and in vivo QA

Abstract

Introduction 3D EPID-based dosimetry can play a key role in the chain of verification procedures in VMAT treatments. Anatomical-based metrics could improve evaluation through meaningful clinical dosimetric parameters both for pre-treatment and in vivo QA. Purpose To evaluate the use of a commercial software to reconstruct dose from EPID measurements for pre-treatment verification and transit dosimetry using 3D patient-based metrics. Materials and methods Dosimetry Check is used for pre-treatment and in vivo 3D evaluations applied to ten VMAT treatments for various anatomical sites (prostate, head & neck, spine and abdomen), planned by Eclipse and delivered by Clinac 2100CD, equipped with a S1000 EPID. Furthermore, for each case, a verification plan is delivered to an homogeneous elliptical shaped solid water phantom to compare in vivo-phantom dose reconstruction and dose measurements by a micro ionization chamber CC01. Results Whole dose grid 3D gamma evaluation is performed with criteria 3%-3 mm and 5%-3 mm. Gamma metrics is applied to specific ROI of interest: PTV, CTV and OARs. DVHs are compared in terms of D2, D98, D50 and OARs constraints values used during planning. Differences between pre-treatment and in vivo reconstructed dose are mostly within 3% (5% in sites with small inhomogeneities). Isodose levels reconstructed from Dosimetry Check in phantom are within 3% compared to absolute dose measured by micro ionization chamber. Conclusion Patient-based metrics can be used to evaluate results in terms of target coverage and OAR constraints, improving QA evaluation for VMAT treatments. Further studies are needed to evaluate action levels for ROI-based gamma evaluation and critical DVH goals.