リニアック 10 MV X 線における肺の線量補正

Abstract

In radiation therapy of thorax region, it is necessary to apply absorbed dose correction for heterogeneity tissuse of lungs. Practically, however, it is not easy to do an accurate absorbed dose correction. We made a basic experiment for absorbed dose correction of heterogeneity tissue of lungs using thorax phantom for 10 MV X-rays, and evaluated the accuracy of depth dose calculation algorithms for its tissue which are available in conventional, commercial-based treatment-planning systems. Depth dose on the central axis in lung for small field of 5×5(cm)^2 was reduced by about 10% due to loss of lateral electronic equilibrium. Dose correction factor of heterogeneity was insensitive to the thickness of the build-up layer of this phantom if there was a longitudinal electronic equilibrium. The equivalent TAR (tissue-air ratio) method gave the greatest accuracy for dose calculation algorithms, of lungs whose error was within about 3% compared to the measured values. However, for the smaller field of 5×5(cm)^2, all our methods did not consider the effects of electron scattering, so that the equivalent TAR method and the ratio of TAR method gave errors of about 15% and the power law TAR method about 6%.