SOME TREATMENT PLANNING CONSIDERATIONS FOR PD-103 AND I-125 PERMANENT INTERSTITIAL IMPLANTS

Sealed sources of palladium-103 (Pd-103), which decay with a half life of 17 days and emit on average 21 keV photons, are now in clinical use for permanent implants. For seed implantation of prostatic cancer, Pd-103 implants are usually planned to deliver 115 Gy to full decay at an initial dose rate of 19.7 cGy/hr whereas I-125 implants are usually planned to deliver 160 Gy at an initial dose rate of 7.72 cGy/hr. Because of the lower energy of photons emitted by Pd-103 compared to the I-125 sources (27 keV average energy), the tissue attenuation is more severe for Pd-103 sources. The radial dose function drops more steeply with distance from the Pd-103 sources compared to the I-125 sources, raising a concern about the possibility of cold spots in the tumors implanted with Pd-103 sources. To investigate this issue, a detailed analysis of the dependence of dose uniformity as a function of seed spacing for I-125 and Pd-103 sources in various cubic and spherical configurations was carried out. Using the measured single source dosimetry data as input, dose distributions for a variety of cubic and spherical implants were generated on a computerized treatment planning system. This study indicates that relative dose distributions for I-125 and Pd-103 implants with the same geometric configuration and number of seeds are very similar inside the implanted volume for implants. Dose uniformity within a target volume implanted with Pd-103 seeds is also very similar to that for I-125. To expedite clinical implementation of Pd-103, an atlas of dose distributions for Pd-103 implants has been produced for various seed configurations, seed spacings, and target volumes. Using I-125 implants as a guideline, clinical procedures for planning of Pd-103 implants have been developed. It was found that the total source strength implanted divided by the dimension of the implant can be expressed as an exponential function of implant size, resulting in a simple method for estimating the strength of seeds necessary in an implant. Also, the air kerma strength of Pd-103 seeds is about 3.3 times that of I-125 sources in an implant with the same geometric configuration and number of seeds, provided treatment doses of 115 Gy and 160 Gy are chosen for Pd-103 and I-125 implants, respectively.