SU-E-T-75: Accurate Dose Measurements with EBT2 Film in Heterogeneous Media

Abstract

Purpose: The confirmation of accuracy on the radiation dosimetry is important to assure the accuracy of the state-of-the-art radiotherapy. However the percent depth dose (PDD) measured by EBT2 film showed abnormal behavior in a heterogeneous phantom compared with Monte Carlo calculation. Thus, we developed a calibration protocol for EBT2 film dosimetry in the heterogeneous medium and applied to evaluate the accuracy of radiotherapy treatment planning system (RTPs). Methods: PDDs were measured with EBT2 film in a polystyrene phantom (20×20×20 cm3) with a small Teflon cube (2×2×2, 3×3×3 cm3) inserted at the center. 6 MV X-rays (Primus, Siemens, USA) with beam size of 2×2, 3×3, 5×5 cm2 were irradiated to the phantom. Dose distributions in the phantom were calculated by Monte Carlo (BEAMnrc, PMCEP code) and compared to theEBT2 film measurements. The Affine Transformation was used for the calculation of calibration matrix which converts dose from EBT2 to MC calculated values in the Teflon cubes and subsequent layer. Identical measurements with EBT2 were conducted for Novalis TX (6 MV) and were compared with predictions by Pinnacle, Eclipse and iplan MC algorithm. Results: EBT2 film data in the Teflon and subsequent layer differed from MC computation by 14.91%∼22.60% and 3.22%∼10.67% in the Teflon and subsequent layer, respectively. Applying the calibration matrix reduced the difference dramatically to 1.10%∼ 5.25 % and 1.26% ∼6.84% in the Teflon and the subsequent layer. Excluding beam size of 5×5 cm2 reduced the deviation below 2.5%. RTPS predicted dose in the Teflon were differentfrom the measurements with EBT2 by 8.7%∼11.6%, 9.3%∼12.3%, and 6.6%∼9.9% in Pinnacle, Eclipse, and iplan, respectively. Conclusions: EBT2 film did not measure the absorbed dose accurately in a high atomic number and dense medium. Developed calibration tool is able to use EBT2 film for dosimetry in the heterogeneous media This research was supportedby “Korea Research Institute of standards and Science”