Dosimetry of high dose rate intracavitary radiation therapy using Co-60 and Ir-192 sources

For decades, the high dose rate sources of Co-60 and Ir-192 have been used in remote controlled afterloading brachytherapy. A small volume ion chamber was used to measure the dose rate in varying distances from the sources. The entire dose distribution around the source was tabulated and the data were used to calculate the treatment time for individualized patient setting. A solid diode detector was used to plot the relative distance dose and the symmetrical dose distribution of the sources. Thermoluminescence dosimetry (TLD) was used for spot-checking of the dose. The dose distribution in the clinically useful range of 2-5 cm was carefully estimated. When normalized at 2.0 cm from the source, the dose rate at 5.0 cm was 17.1% for the Co-60 source and 18.4% for the Ir-192 source. It is clearly demonstrated that Ir-192 offers no dosimetric advantages, which is contrary to what used to be believed by some clinicians. The parameters of the computer treatment planning system have to be corrected and then adopted into clinical application.