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Some treatment planning considerations for103Pd and125I permanent interstitial implants

Authors
Publisher
Elsevier Inc.
Publication Date
Volume
22
Issue
5
Identifiers
DOI: 10.1016/0360-3016(92)90820-8
Keywords
  • Palladium-103
  • Iodine-125
  • Brachytherapy
  • Interstitial Implants
  • Permanent Implants
  • Dosimetry
  • Dose Calculations
  • Treatment Planning
Disciplines
  • Mathematics
  • Physics

Abstract

Abstract 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.

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