TLD, DIODE AND MONTE-CARLO DOSIMETRY OF AN IR-192 SOURCE FOR HIGH-DOSE-RATE BRACHYTHERAPY

Very few dosimetry data are available for the current generation of hi gh-dose-rate (HDR) Ir-192 sources, which have broad application in rem otely afterloaded brachytherapy. We have measured the two-dimensional dose rate distribution around a microSelectron-HDR source and used the results to validate Monte Carlo simulations. Thermoluminescent dosime ters (TLDs) in solid-water phantoms were used to measure the transvers e-axis dose rates in the distance range 0.5-10 cm and the polar dose-r ate profiles at 1.5, 3 and 5 cm distance from the source. At close dis tances, 2-40 mm from the HDR source, we performed transverse axis dose -rate measurements with a Si diode in water. We performed diode measur ements at the same distances also for a pulsed dose-rate (PDR) source to compare the results for Ir-192 sources With different encapsulation . Both the HDR and the PDR sources were decayed, separated from their cables and calibrated prior to the measurements. The measured dose rat es were compared with Monte Carlo photon transport calculations, which realistically modelled the experimental and source geometry at each m easurement point. Agreement between Monte Carlo photon transport absol ute dose-rate calculations and measurements was, on average, within 5% . From the transverse-axis experimental data, we deduced a value for t he dose-rate constant Lambda(0) of Ir-192 HDR sources of 1.14 cGy h(-1 ) U-1 +/- 5%. This value agrees within the experimental error with the Monte Carlo estimate of 1.115 cGy h(-1) U-1 +/- 0.5%. Excellent agree ment with previously measured anisotropy functions was observed. Highe r anisotropy is observed for the point at 0 degrees along the source c able for which no previous data have been reported.