In vivo and phantom measurements versus Eclipse TPS prediction of near surface dose for SBRT treatments

This study reports on Gafchromic EBT2 film skin dose measurements for lung stereotactic body radiotherapy. These measurements were compared to near surface skin doses predicted by Eclipse treatment planning system (TPS) using the Analytical Anisotropic Algorithm (AAA) for a 6 MV photon beam. The accuracy of the predicted near surface dose for 3×3, 5×5 and 10×10 cm² fields was assessed using an Attix chamber and EBT2 film in a Virtual Water phantom and compared to Monte Carlo calculation. The maximum near surface dose and its location were identified from the patient's treatment plan. For phantom measurements, the TPS dose (nominal 0 mm depth) was higher than the Attix chamber data by up to 24.0 % but in closer agreement with the EBT2 film measurement, which was up to 10.3 % higher than the Attix data. The MC calculated dose values were higher than the Attix data by up to 3.5% for the depths up to 2 mm. The maximum patient skin dose estimated from in vivo EBT2 film measurements was 7.5 – 19.5 Gy per course and depended on the number of overlapping fields, beam weight and/or contact with immobilisation devices. The TPS predicted dose for patient plans was mostly higher than the in vivo dose, by as much as 69.3%, but in two cases was lower, by as much as -23.1%.