IN-WATER CALIBRATION OF PDR IR-192 BRACHYTHERAPY SOURCES WITH AN NE2571 IONIZATION-CHAMBER

An ionometric calibration procedure for Ir-192 PDR brachytherapy sourc es in terms of dose rate to water is presented. The calibration of the source is performed directly in a water phantom at short distances (1 .0, 2.5 and 5.0 cm) using an NE2571 Farmer type ion chamber. To conver t the measured air-kerma rate in water to dose rate to water a convers ion factor (CF) was calculated by adapting the medium-energy, x-ray do simetry protocol for a point source geometry (diverging beam). The obt ained CF was verified using two different methods. Firstly, the CF was calculated by Monte Carlo simulations, where the source-ionization ch amber geometry was modelled accurately. In a second method, a combinat ion of Monte Carlo simulations and measurements of the air-kerma rate in water (at 1.0, 2.5 and 5.0 cm distance) and in air (1 m distance) w as used to determine the CF. The obtained CFs were also compared with conversion factors calculated with the adapted dosimetry protocol for high-energy photons introduced by Tolli. All calculations were done fo r a Gammamed PDR Ir-192 source-NE2571 chamber geometry. The conversion factors obtained with the four different methods agree to within 1% a t the three distances of interest. We obtained the following values (m edium-energy x-ray protocol): CF(1 cm) = 1.458; CF(2.5 cm) = 1.162; CF (5.0 cm) = 1.112 (1 sigma = 0.7% for the three distances of interest). The obtained results were checked with TLD measurements. The values o f the specific dose rate constant and the radial dose function calcula ted in this work are in accordance with the literature data.