MEASUREMENT AND CALCULATION OF HETEROGENEITY CORRECTION FACTORS FOR AN IR-192 HIGH-DOSE-RATE BRACHYTHERAPY SOURCE BEHIND TUNGSTEN ALLOY ANDSTEEL SHIELDS

Shields made of high atomic number material are commonly used in vagin al applicators with high dose-rate (HDR) (192)IR remotely afterloaded brachytherapy sources. However little data is available for the dose d istribution around such shields. Heterogeneity correction factors (HCF s) are defined as the ratio of the dose to a point with the heterogene ity (shield) in place, divided by the dose to the same point with no h eterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten al loy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity cor rection factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between m easured and calculated HCFs on the average was within 6%. Tungsten all oy disks resulted in about two times greater dose reduction in water ( HCF approximate to 0.4, for 20X4 mm disk) than for a steel disk with t he same dimensions (HCF approximate to 0.85). Reducing the disk diamet er to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. Th e detector artifact effects arising from the finite size and different composition of the TLD chips were determined. (C) 1996 American Assoc iation of Physicists in Medicine.