Abstract
Prostate low-dose-rate (LDR) brachytherapy involves implantation of radioactive seeds permanently into the prostate gland. The International Commission on Radiological Protection (ICRP) provides guidance on radiation precautions based on potential external exposures to members of the public and/or carers for prostate seed brachytherapy. During receptive anal intercourse (RAI) the penis of the partner may come in close proximity to the implanted prostate gland. Here, we estimate the potential intra-rectal dose rates in prostate cancer patients treated with LDR brachytherapy. One hundred and two patients were included in the study. At the end of the seed implantation procedure, while the patients under anesthesia in the dorsal lithotomy position a new set of ultrasound (US) images and a CT scan were obtained. The images were fused, radioactive seeds and US probe locations were determined on the CT, and prostate, bladder and rectal contours were drawn on the US for dosimetry. Dose rates (cGy/h) were calculated for the portion of the US probe spanning the prostate for several DVH parameters. Twenty patients were treated with Iodine-125, and 82 patients with Palladium-103. At day 0, maximal and average dose rates per hour to the US probe for patients treated with 125I were 6.7±4.8cGy/hr and 2.1±1.3cGy/hr, and for 103Pd were 10.3±4.0cGy/hr and 2.5±0.8cGy/hr, respectively. After 60 days, average calculated probe dose dropped to 1.0 ±0.6cGy/hr and 0.2 ±0.1cGy/hr, for 125I and 103Pd, respectively. During the immediate weeks after prostate seed implant, the estimated intra-rectal dose rates are higher in 103Pd compared to 125I. Both would likely exceed equivalent ICRP guidance, after limited RAI time. As 103Pd decays faster than 125I, two months after the implant, radiation exposure from 103Pd becomes lower than 125I. RAI should be avoided for 2 and 6 months after LDR brachytherapy of the prostate with 103Pd and 125I, respectively.
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