Calculation of the dose equivalent around a patient receiving treatment with 125I seeds

Interstitial brachytherapy with 125I seeds for treatment of prostate cancer is being carried out successfully in Europe and U.S.A. However, its widespread use in Japan has been limited by regulations governing the exposure of individuals. Basic radiation protection data are required to promote the use of 125I seed sources. In preparation for implementing this new modality, we carried out a series of measurements to determine the 1 cm dose equivalent in a caregiver located 1 m from the implanted patient. These measurements were compared with published recommendations of acceptable doses, and may be used to develop guidelines for discharge of the patient. The 1 cm dose equivalent was measured 1 m from the source under clinically relevant conditions by placing 50 125I seeds (437.5 MBq) into the portion of a humanoid phantom that corresponds to the prostate. The 1 cm dose equivalent was 0.0014 μSv·m2·MBq-1·h-1 1 m from the surface of the phantom. The calculated dose to a caregiver based on this figure is well below the 5 mSv value recommended by the IAEA as a constraint dose for the caregiver. These measurements and calculations suggest that 125I seed implants of outpatients should be permissible. (author)