Dose Alterations at the Distal Surface by Tissue Inhomogeneity in High Energy Phonton Beam

摘要

PURPOSE: This study was performed to measure dose alteration at the air-tissue interface resulting from rebuild-up due to the loss of charged particle equilibrium in the tissues around the air-tissue interfaces. MATERIALS AND METHODS: The 6 and 10-MV photon beam in dual energy linear accelerator were used to measure the surface dose at the air-tissue interaface. The polystyrene phantom sized 25 X 25 X 5 cm3 and a water phantom sized 9 X 29 X 48 cm3 which incorporates a parallel-plate ionization chamber in the distal side of air gap were used in this study. The reatment field sizes were 5 X 5 cm2 , 10 X 102 , and 20 X 20 cm2 . Air cavity thickness was variable from 100 mm to 50 mm. The observed-expected ratio (OER) was defined as the ratio of dose measured at the distal junction, that is air-tissue interface, to the dose measured at the same point in a homogeneous phantom. RESULTS: In this experiment, the result of OER was close or slightly over than 1.0 for the large field size but much less (about 0.565) than 1.0 for the small field size in both photon energy. The factors to affect the dose distribution at the air-tissue interface were the filed size, the thickness of air cavity, and the photon energy. CONCLUSION: Thus, the radiation oncologist should take into account dose reduction at the air-tissue interface when planning the head and neck cancer, especially pharynx and laryngeal lesions, because the dose can be less nearly 29% than predicted value.