Verification of air-kerma strength of ¹²⁵I seed for permanent prostate implants in Japan

Background: To assure the physical quality of brachytherapy, we investigated the difference between measured and manufacturer's stated source strengths in a single model SourceTech Medical (STM)1251 ¹²⁵I seed. Methods: A well-type ionization chamber with a single-seed holder was used to measure the source strength of 2412 ¹²⁵I seeds before implant in 34 patients. The air-kerma strength was 0.450 U for all cases. The mean source strength for each patient was measured and compared with the manufacturer's stated value. The deviation from the measured value was compared with the tolerance range of the American Association of Physicists in Medicine (AAPM) TG-56 report's recommendation. Results: The measured source strength was higher than the manufacturer's stated value, with a median difference of 1% (range, 2% to 5%). Sixteen of the total of 2412 seeds (0.7%) were more than 5% different from the manufacturer's stated value. The median SD from the mean value was 2.2% (range, 1.1% to 2.5%) for all patients. Conclusion: This is the first report of a single-seed assay performed for the model STM1251 ¹²⁵I seed. In this study the manufacturer's stated strength agreed well with the measured value. Nevertheless, the advisability of performing a single-seed assay at every institution should be considered, by referring to the appropriate regulations; for example, those used in the United States.