124. Evaluation of dose calculation accuracy at lung-tissue interface in presence of magnetic field with Gafchromic EBT3

Abstract Purpose Aim of this study is to investigate the MRIdian (ViewRay) TPS accuracy in calculating the dose distribution in presence of magnetic field at tissue-lung interface, where the electron return effect (ERE) plays a primary role. Methods The investigation was realised using Gafchromic EBT3 films. A dose calibration step was planned on MRIdian, exposing thirteen 3 × 3 cm2 film pieces perpendicularly to the beam axis at different Co-60 source exposure times (15–460 s). Films were arranged in a slab water phantom (SAD = 105 cm; SSD = 100 cm) and analysed one day after irradiation. An absolute dose measurement was performed by using a 125 cc ion chamber to estimate correlation between exposure time and absorbed dose. To evaluate dose accuracy at lung-tissue interface, two EBT3 films were inserted at the interfaces of a non-homogeneous sandwich phantom composed by a 5 cm lung-equivalent slab (ρ = 0.39 g/cm3) embedded inside two water-equivalent slabs (see Fig. 1). Three treatment plans were calculated and delivered: • Plan A: one 4.2 × 4.2 cm2 beam delivered at 0 degree • Plan B: three 6.3 × 6.3 cm2 beams delivered at 330,0 and 30 degree • Plan C: an IMRT complex plan constituted by 9 equidistant beams The dose distributions measured by EBT3 films were compared to those calculated by TPS in terms of 1%/1 mm, 3%/1 mm, 3%/3 mm and 25 cGy/1 mm gamma analysis. The dosimetric workflow was also validated by delivering Plan A on a totally homogeneous phantom. Results Table 1 contains the γ 1 values obtained for the different plans delivered. All plans show a 3%/3 mm gamma-passing rate higher than 90% (tolerance criteria recommended for IMRT by ESTRO Booklet 7). When tighter spatial criteria were considered (delta distance = 1 mm), a decrease of passing rate is observed, whose entity is correlated to the plan complexity. Conclusions MRIdian TPS is able to calculate with high accuracy dose distribution in presence of magnetic field at the tissue-lung interface, correctly modelling the ERE contribution.