PUB110 Clinical Results of an EPID-Based In-vivo Dosimetry for Lung Cancer Radiation Therapy

Abstract

In-vivo dose verification is a step of quality assurance to ensure that the dose delivered during treatment agrees with the prescribed. This work reports the in-vivo dosimetry (IVD) results obtained with SOFTDISO (Best Medical Italy). SOFTDISO was developed by National Institute of Nuclear Physics and Catholic University of Sacred Heart (Rome). The software reconstructs in quasi-real time the dose at isocenter (Diso) from the transit signal acquired by the EPID and the comparison between EPIDs of every fractions. For each beam and fraction, the R ratios between Diso in 3D-CRT plans for lung targets and the dose (2 Gy/fraction) calculated by the treatment planning system Diso,TPS (Oncentra) were computed. The acceptance criteria was 0.95≤R≤1.05. The γ-analysis (DD=5%; Dd=5mm) between EPIDs to test the beam delivery reproducibility with the Pγ<1>90% and γmean<0.5 was performed. Fourteen patients with lung cancer were treated with 6 MV photon beams delivered by an Elekta Synergy Agility. The pass criteria was Δd equal to the mean distance Δr (mm) along x, y, z directions, accepted in the patient set-up reproducibility. The IVD test were performed twice weekly when the CBCT wasn't acquired. The IVD supplied 682 tests. The tests off-tolerance were >13% for R, >35% for Pγ<1 and >16% for γmean. For each patient the Rmean value was within the tolerance level. As regards Pγ<1 and γmean, 70% and 86% of patients had values within the tolerance, respectively. These values were obtained considering set-up errors and morphological changes (tumor reduction, atelectasis and/or air). Results of an EPID-based in-vivo dosimetry for lung cancer treatments are in tolerance for the dose at the isocenter (Rvalue). The γ-analysis suffer from some restrictions due to morphological changes that affect the results. The SOFTDISO real-time analysis allows to daily investigate the dose delivery.