AbstractID: 7451 Title: The measurements of the interface dose on heterogeneous tissues
The measurements of the interface dose on heterogeneous tissues
Purpose: In clinic, we apply correction factors to correct dose for inhomogeneous tissue. In this
experiment we measured dose distribution on heterogeneous tissues using TLD dosimeters. We
compared the calculated dose values from treatment planning system with the measured dose values
for tissue interface.
Method and Materials: We manufactured tissue equivalent phantom with three different densities
for this experiment. We placed TLD in the predetermined position on each phantom and on the
interface to measure the absolute dose with 6MV and 18 MV x-ray. We simulated the experimental
setup for treatment planning system to obtain calculated dose values. Finally, we analyzed the
results.
Results: The results of comparison for soft-tissue only phantom show some differences in the
buildup region, but almost no difference after dmax. The results of comparison on heterogeneous
tissues, lung and soft tissue, show a small difference about 3~5% for 6MV and about 4~6% for
18MV(fig1). The results of comparison on soft tissue-bone-soft tissue phantom show the calculated
value that are greater than measurement values by 3~7%(fig2).
Conclusions and discussions: The calculation model is confirmed by the agreement for
comparisons on the homogeneous phantom. The reason of the difference of dose values on
heterogeneous tissues are that the algorithm used in treatment planning system (FOCUS) is not
perfect. This algorithm only takes into account of attenuation effect and but not scatter effect. It
could cause insufficient dose in clinical application.
Key words: heterogeneous tissues, tissue interface, dose calculation