Clinical problems associated with dosimetry of 125-iodine for cancer of the prostate

Dosimetry and clinical data for iodine-125 prostatic seed implants in 63 patients were retrospectively studied to evaluate the methods of calculating the effective implant activity. Our protocol called for 10 000 cGy to the periphery of the prostate from 125I and 4 000 cGy to the whole pelvis in 20 fractions from supervoltage. Most procedures used seed strengths of 20.35 to 21.09 MBq (0.55-0.57 mCi). The average number of seeds lost per patient was 2.1 without prior transurethral prostatic resection (TURP), and 5.6 with prior TURP. Specific precautions are recommended to minimize seed loss. In a 3 cm diameter gland with uniform seed distribution, the periphery received 10 000 cGy, while the central 2 cm diameter core received 20 000 cGy. Near the periphery there was a rapid decrease in dose averaging 1 390 cGy per millimeter radially. Because of this rapid fall-off, it may be misleading to specify the dose in cGy only. It is recommended that the total 125I activity per implant also be specified in the same way as mghr-of-radium-equivalent for cancer of the cervix. Recommended total activities needed for implantation in various prostate gland sizes are given.