2種類の指頭形電離箱による胸部ファントム内の線量測定

概要

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Tissue-maximum ratios (TMR) in a thorax phantom were measured using two types of chambers for 10 MV X-ray fields at a source-chamber distance (SCD) of 100 cm. One was a Farmer-2581 chamber with a cavity volume of 0.6 cm^3 and wall thickness of 0.04 g/cm^2, and the other was a Capintec-PR-05P with a cavity volume of 0.07cm^3 and wall thickness of 0.22 g/cm^2. It has been found that in the back soft-tissue layer for fields of 5×5 to 15×15 cm^2, the ralative deviation of the TMR value by the 0.6-cm^3 chamber from the one by the 0.07-cm^3 chamber ranges from -0.9% not necessary to +0.5%. No significant difference existed between the two chambers in TMRs within the soft-tissue (Mix-Dp) layers. However, the following results has been obtained in the lung (cork with a density of 0.23g/cm^3) layer: (1) When a wall correction is performed for both chambers, the ralative deviation of the TMR value by the 0.6-cm^3 chamber from the one by the 0.07-cm^3 chamber is about -3% for a field of 5×5 cm^2. (2) The 0.6-cm^3 chamber makes small readings for fields of 6×6 cm^2 and less die to lack of uniformity in the dose distribution yieded in the 0.6-cm^3 chamber cavity. (3) A wall correction is needed for the 0.07-cm^3 chamber for fields of 7×7 cm^2 and less because the chamber wall is relatively thick.
1989-12-01