Is an 192Ir permanent seed implant feasible for prostate brachytherapy?

Purpose: 125I permanent seed brachytherapy for prostate cancer produces good clinical outcomes and limits radiation exposure to medical staff and patients' families. However, 125I seeds cost thousands of dollars per implant. An encapsulated 192Ir permanent seed possibly could cost less than $10. Could inexpensive permanent 192Ir seeds be used for prostate implants? Methods and Materials: We review the radiobiology of permanent implants, calculate the 192Ir permanent seed air kerma strength (activity) required, simulate 125I and 192Ir seed implants and mixtures thereof, calculate exposure rates near simulated 192Ir prostate patients, calculate potential radiation exposure to medical staff and family members, review patient release regulations, and analyze the potential cost benefits of using 192Ir permanent seed implants. Results: Low air kerma strength (<0.4 μGy m2/h/seed) [activity < 0.1-mCi/seed; <0.0558 mg Ra eq/seed] permanent 192Ir seed implants yield more uniform prostate doses than 125I seed implants and acceptable urethra, bladder, and rectal doses. The 192Ir 73.83-day half-life allows mixing 192Ir seeds and 125I seeds. Conclusions: We believe medical staff could safely implant 40 μGy m2/h [10-mCi; 5.58 mg Ra eq] 192Ir per case. Occupancy factors (1/8, 1/16) could acceptably limit families' exposures. Seed costs could be reduced markedly. With adequate protection of medical staff and proper instructions to patients post-implant, low air kerma strength (<0.4 μGy m2/h/seed) [activity <0.1-mCi/seed; <0.0558 mg Ra eq/seed] 192Ir permanent seed implants are feasible in large patients, with mixed (125I, 92Ir) seed implants feasible for modest size patients. Such implants could be useful in populous countries (China, India, Brazil) and for others who find 125I seed implants too expensive to perform.