Quasi real time in vivo dosimetry for VMAT

Purpose The paper reports the results about the feasibility of a method based on the correlation between the EPID signal and the dose on the beam central axis for the in-vivo dosimetry (IVD) at the isocenter point of volumetric modulated arc therapy (VMAT). Moreover a γ-analysis of 2D-EPID images obtained day to day was adopted to verify the plan delivery reproducibility. Materials and methods An algorithm was developed to take into account the dose contributions due to open and closed segments on the beam central axis, delivered by two opposite arcs. Reconstructed doses at the isocenter point for VMAT treatments of 10 prostate tumours showed a good agreement with the planned doses and measured dose in a conic PMMA phantom with elliptical section. Results The average isocenter dose ratio R, between the reconstructed doses in phantom and planned was equal to 1.015 with a range between 0.980 and 1.035. The IVD was applied to 10 prostate treatments and the average ratio resulted equal to...A γ-analysis of EPID images, resulted efficient to test the plan delivery reproducibility. In particular using 5% 3 mm criteria, 95% of the tests supplied pass rates γmean≤0.5 and Pγ<1 ≥90%.