3D Dosimetric verification of volumetric-modulated arc therapy by portal dosimetry

Background and purpose: To demonstrate the feasibility of back-projection portal dosimetry for accurate 3D dosimetric verification of volumetric-modulated arc therapy (VMAT), pre-treatment as well as in vivo. Materials and methods: Several modifications to our existing approach were implemented to make the method applicable to VMAT: (i) gantry angle-resolved data acquisition, (ii) calculation of the patient transmission, (iii) compensation for detector 'flex' and (iv) 3D dose reconstruction and evaluation. Results: Planned and EPID-(Electronic Portal Image Detector)-reconstructed close distributions show good agreement for pre-treatment verification of two prostate, a stereotactic lung and a head-and-neck VMAT plan and for in vivo verification of VMAT treatments of prostate and lung cancer. Averaged over pre-treatment verifications, planned and measured isocentre dose ratios were -1.2% (range [-4.7%,1.8%]). 3D gamma analysis (3% maximum dose, 3 mm) revealed mean gamma <gamma(mean)> = 0.37 [0.34,0.39], maximum 1% gamma (gamma(1%)) = 0.72 [0.66,0.81] and percentage of points with -gamma <= 1 < P(gamma <= 1)> = 99% [97%,100%.] For in vivo verification, the average isocentre dose ratio was -1.2% [-0.8%,-1.7%], <gamma(mean)> = 0.52 [0.40,0.64], <gamma(1%)> = 0.92 [0.76,1.08] and < P(gamma <= 1)> = 96% [93%,100%]. Conclusions: Our portal dosimetry method was successfully adapted for verification of VMAT treatments, pre-treatment as well as in vivo. (C) 2010 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 94 (2010) 181-187