Ultrasound guided I–125 permanent prostate implants: seed calibration and radiation exposure levels

Abstract

Purpose: (1) To present data on I–125 seed calibration in a clinical setting so that reasonable tolerance levels can be set for the discrepancy in seed strength between manufacturer specified value and institution measured value; (2) To present data on measured exposure rates and estimate radiation exposure levels associated with I–125 prostate implants.Methods and Materials: Ten percent of each batch for 50 batches of I–125 seeds were calibrated using an HDR 1000 PLUS well chamber with a single source holder. Exposure rates due to I–125 were measured by survey meters with a scintillation probe designed for low energy photon counting, as well as a survey meter of the ionization chamber type. Exposure rates of an unshielded I–125 seed, a needle loaded with three seeds, and 54 prostate implant patients immediately after the implant were obtained.Results: Compared to the manufacturer stated midrange seed strength for a batch of seeds, the average seed strength of sampled seeds had maximum deviations of ±8%, however for 45 out of 50 batches the deviation was less than ±5%. Measured single seed strength deviated up to ±12% from the manufacturer stated midrange value, and between −11% to 7% from the mean of the sampled batch. The exposure rate of a 1.39×107 Bq (0.375 mCi) unshielded I–125 seed was about 1.548×10−8C/kgh (0.06 mR/h) at 1 m, and 1.29×10−6C/kgh (5 mR/h) at 10 cm. For a needle loaded with three seeds, the exposure rate was 1.29×10−8C/kgh (0.05 mR/h) at the handle, and 1.29×10−7C/kgh (0.5 mR/h) along the shaft. For patients implanted with I–125 seeds in the prostate, the average exposure rate was 3.61×10−8C/kgh (0.14 mR/h) at 1m, and 4.13×10−7C/kgh (1.6 mR/h) at the pelvis surface.Conclusions: For the mean seed strength a first action level should be set at a deviation of at least 5% deviation from the manufacturer stated midrange value. For individual seeds, a first action level set at 10% deviation from the manufacturer stated midrange value seems reasonable. A person performing I–125 seed calibration or seed loading could receive up to 0.5 mSv (50 mR) per case to the hands. In the first year following an I–125 prostate implant, the spouse of the patient could receive slightly over 1mSv from the I–125 in the patient. A co-worker should not receive more than 0.5 mSv from the patient.