고에너지 광자선을 이용한 방사선 치료 시 소조사면에서의 흡수선량평가에 관한 연구

초록

In case of radiation treatment using small field high-energy photon beams, an accurate dosimetry is a challenging task because of dosimetrically unfavorable phenomena such as dramatic changes of the dose at the field boundaries, dis-equilibrium of the electrons, and non-uniformity between the detector and the phantom materials. In this study, the absorbed dose in the phantom was measured by using an ion chamber and a diode detector widely used in clinics. GAFCHROMIC(R) EBT films composed of water equivalent materials was also evaluated as a small field detector and compared with ionchamber and diode detectors. The output factors at 10 cm depth of a solid phantom located 100 cm from the 6 MV linear accelerator (Varian, 6 EX) source were measured for 6 field sizes (5×5cm2, 2×2cm2, 1.5×1.5cm2, 1×1cm2, 0.7×0.7cm2 and 0.5×0.5cm2). As a result, from 5×5cm2 to 1.5×1.5cm2 field sizes, absorbed doses from three detectors were accurately identified within 1%. Wheres, the ion chamber underestimated dose compared to other detectors in the field sizes less than 1×1cm2. In order to correct the observed underestimation, a convolution method was employed to eliminate the volume averaging effect of an ion chamber. Finally, in 1×1cm2 field the absorbed dose with a diode detector was about 3% higher than that with the EBT film while the dose with the ion chamber after volume correction was 1% lower. For 0.5×0.5cm2 field, the dose with the diode detector was 1% larger than that with the EBT film while dose with volume corrected ionization chamber was 7% lower. In conclusion, the possibility of GAFCHROMIC(R) EBT film as an small field dosimeter was tested and further investigation will be proceed using Monte Calro simulation.

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