TLD, diode and Monte Carlo dosimetry of an ¹⁹²Ir source for high dose-rate brachytherapy

Very few dosimetry data are available for the current generation of high-dose-rate (HDR) ¹⁹²Ir sources, which have broad application in remotely afterloaded brachytherapy. The authors have measured the two-dimensional dose rate distribution around a microSelectron-HDR source and used the results to validate Monte Carlo simulations. Thermoluminescent dosimeters (TLDs) in solid-water phantoms were used to measure the transverse-axis dose rates in the distance range 0.5-10 cm and the polar dose-rate profiles at 1.5, 3 and 5 cm distance from the source. At close distances, 2-40 mm from the HDR source, the authors performed transverse axis dose-rate measurements with a Si diode in water. They performed diode measurements at the same distances also for a pulsed dose-rate (PDR) source to compare the results for ¹⁹²Ir, 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 rates were compared with Monte Carlo photon transport calculations, which realistically modelled the experimental and source geometry at each measurement point. Agreement between Monte Carlo photon transport absolute dose-rate calculations and measurements was, on average, within 5%. From the transverse-axis experimental data, the authors deduced a value for the dose-rate constant Lambda ₀ of ¹⁹²Ir HDR sources of 1.14 cGy h^-1 U^-1+or-5%. This value agrees within the experimental error with the Monte Carlo estimate of 1.115 cGy h^-1 U^-1+or-0.5%. Excellent agreement with previously measured anisotropy functions was observed. Higher anisotropy is observed for the point at 0 degrees along the source cable for which no previous data have been reported.