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Accurate Dose Measurements with EBT2 Film in Heterogeneous Media

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H JUNG1*, O Kum2, J Shin3, Y Han4*, (1) SungKyunKwan University, Seoul,Korea (2)KyunYang University Medical Center,DAEJON, ,(3) Samsung Medical Center,Seoul ,Korea,(4) Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul,Korea

SU-E-T-75 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

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 (20x20x20 cm³) with a small Teflon cube (2x2x2, 3x3x3 cm³) inserted at the center. 6 MV X-rays (Primus, Siemens, USA) with beam size of 2x2, 3x3, 5x5 cm² were irradiated to the phantom. Dose distributions in the phantom were calculated by Monte Carlo (BEAMnrc, PMCEP code) and compared to the EBT2 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 5x5 cm² reduced the deviation below 2.5%. RTPS predicted dose in the Teflon were different from 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

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