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KMID : 0384620090200040290
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Korean Journal of Medical Physics
2009 Volume.20 No. 4 p.290 ~ p.297
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Study on the Small Fields Dosimetry for High Energy Photon-based Radiation Therapy
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Jeong Hae-Sun
Kim Chan-Hyeong
Han Young-Yih
Kum O-Yeon
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Abstract
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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. 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 (, , , , and ). As a result, from to 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 . In order to correct the observed underestimation, a convolution method was employed to eliminate the volume averaging effect of an ion chamber. Finally, in 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 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 EBT film as an small field dosimeter was tested and further investigation will be proceed using Monte Calro simulation.
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KEYWORD
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Small field, Absorbed dose, GAFCHROMINⓡ EBT, Ion chamber, Diode detector
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