Some treatment planning considerations for ¹⁰³Pd and ¹²⁵I permanent interstitial implants

Sealed sources of palladium-103 (¹⁰³Pd), 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, ¹⁰³Pd implants are usually planned to deliver 115 Gy to full decay at an initial dose rate of 19.7 cGy/hr whereas ¹²⁵I 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 ¹⁰³Pd compared to the ¹²⁵I sources (27 keV average energy), the tissue attenuation is more severe for ¹⁰³Pd sources. The radial dose function drops more steeply with distance from the ¹⁰³Pd sources compared to the ¹²⁵I sources, raising a concern about the possibility of cold spots in the tumors implanted with ¹⁰³Pd sources. To investigate this issue, a detailed analysis of the dependence of dose uniformity as a function of seed spacing for ¹²⁵I and ¹⁰³Pd 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 generatled on a computerized treatment planning system. This study indicates that relative dose distributions for ¹²⁵I and ¹⁰³Pd 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 ¹⁰³Pd seeds is also very similar to'ithat for ¹²⁵I. To expedite clinical implementation of ¹⁰³Pd an atlas of dose distributions for ¹⁰³Pd implants has been produced for various seed configurations, seed spacings, and target volumes. Using ¹²⁵I implants as a guideline,; clinical procedures for planning of ¹⁰³Pd 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 l of ¹⁰³Pd seeds is about 3.3 times that of 1251 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 ¹⁰³Pd and ¹²⁵I implants, respectively.