Dosimetry of Ir-192 sources used for endovascular brachytherapy

An in-phantom calibration technique for Ir-192 sources used for endovascula r brachytherapy is presented. Three different source lengths were investiga ted. The calibration was performed in a solid phantom using a Farmer-type i onization chamber at source to detector distances ranging from I cm to 5 cm . The dosimetry protocol for medium-energy x-rays extended with a volume-av eraging correction factor was used to convert the chamber reading to dose t o water. The air kerma strength of the sources was determined as well. EGS4 Monte Carlo calculations were performed to determine the depth dose distri bution at distances ranging from 0.6 mm to 10 cm from the source centre. In this way we were able to convert the absolute dose rate at 1 cm distance t o the reference point chosen at 2 mm distance. The Monte Carlo results were confirmed by radiochromic film measurements, performed with a double-expos ure technique. The dwell times to deliver a dose of 14 Gy at the reference point were determined and compared with results given by the source supplie r (CORDIS). They determined the dwell times from a Sievert integration tech nique based on the source activity. The results from both methods agreed to within 2% for the 12 sources that were evaluated. A Visual Basic routine t hat superimposes dose distributions, based on the Monte Carlo calculations and the in-phantom calibration, onto intravascular ultrasound images is pre sented. This routine can be used as an online treatment planning program.