High Source Strength of Most Blister Packs Delivered for 125I Seed Brachytherapy: A Case Report

Since the beginning of 125I seed brachytherapy, our facility has verified the source strength of all blister packs delivered. We set a reference range based on measurements taken with a well-type ionization chamber. There are scattered reports on verification for the purpose of detecting dead seeds, but to our knowledge, there are no reports on the application of source strength verification results in actual clinical practice for deviations of a few percent below the intervention value. In this case, five of the six TheraAgX100 (Theragenics Corporation, Buford, GA, USA) blister packs delivered had a source strength that was 3-5% stronger than the nominal value. We report on this case, in which we consulted with the physician regarding the placement of the source, taking into consideration the measured source strength, which led to a safer treatment. We used VariSeed 9.0 (Varian Medical Systems Co., Ltd., Palo Alto, CA, USA), a treatment planning system (TPS), with a nominal value of 0.274 mCi and a nominal value of 0.288 mCi, which is 5% stronger than the nominal value for safety reasons, and verified the difference in prescribed doses for planning target volume (PTV) and organ at risk (OAR). We placed 0.288 mCi in areas where the distance from the urethra and rectum, which are susceptible to adverse events, could be ensured. When comparing the nominal value and measured value of the prescribed dose, the measured value of PTV D₉₀ increased by 2.73%. In addition, as indicators of the dose to the urethra, UD₁₀ and UD₃₀ increased by 3.04% and 2.64%, respectively, in the measured values. Next, as an indicator of the dose to the rectum, RD_{0.1 cc} increased by 3.43 Gy in the measured values. In radiation therapy, it is essential to accurately determine the prescribed dose. Evaluating the deviation in source strength in relation to the prescribed dose is not only useful for determining the effectiveness of treatment, but also enables the source to be left in place while minimizing the occurrence of adverse events. It is an important responsibility of medical physicists to provide the best possible treatment to patients by thoroughly conducting the verification that has been advocated and collaborating with physicians.