Measurement of 1 cm dose equivalent rate for a patient treated with ¹²⁵I prostate brachytherapy

The purpose of this study was to quantify 1 cm dose equivalent rate of patients with localized prostate cancer treated with ₁₂₅I seed implantation, to evaluate predictive factors for the measured dose and to assess radiation safety to the general public. Ionization chamber, which is authorized to detect the photons with an energy lower than 30 keV, was used to measure the photons emitted by I-125 with a mean energy of 21.4 keV. Measurements of the 1 cm dose equivalent rate were taken at one meter perpendicular from the skin surface in the anterior, lateral and caudal directions in the supine position for 70 patients treated with ¹²⁵I seed implantation. As a result, the 1 cm dose equivalent rate measured from the anterior direction was the highest and correlated with the depth of the prostate from the patient's surface. In 7 patients, the 1 cm dose equivalent rate at 1 meter from the anterior skin surface exceeded 1.8 μ Sv/h of the limitation of discharge criteria, but their total seed activity was less than 1,300 MBq. Their depth of prostate from the skin surface was thin compared with the depth of the remaining patients, whose dose rate was 1.8 μ Sv/h or less. This result gives a warning that the 1 cm dose equivalent rate from a thin patient could exceed 1.8 μ Sv/h even if a patient contains less overall seed activity than the limit of the criteria, so adequate instructions are needed to minimize the exposure to the public. The estimated value of the patient's peripheral dose equivalent rate from our data was 0.0008 μ Sv·m ²/ MBq/h, which was much lower than the effective dose rate constant of 0.0014 μ Sv·m ²/MBq/h adopted as a evaluation of radiation exposure of ¹²⁵I seed source in Japan. The mean value of the 1 cm dose equivalent rate per 1 MBq at 1 meter from the anterior skin surface was 0.0015 μ Sv·m ²/MBq/h, which was nearly equal to the effective dose rate constant adopted in Japan. Therefore, these data suggest that radiation safety to the general public is kept with in Japanese discharge criteria calculated from the effective dose rate constant of ¹²⁵I seed source.