Regression Equation for Predicting Source Strength Detection Rate in Iodine-125 Seed Brachytherapy

Prostate cancer is one of the most common malignant tumors in males, and its incidence is increasing worldwide. In Japan, iodine-125 (125I) seed brachytherapy was introduced in 2003 and has since been used to treat many patients. Iodine-125 (125I) seed brachytherapy is a treatment in which multiple small capsules containing 125I are implanted into the prostate. Naturally, if the activity of the 125I source differs from the manufacturer's stated value, it may affect treatment outcomes. European and American guidelines state that it is the responsibility of medical physicists to verify the source activity before treating patients. In previous studies, there have been three cases in Japan where seeds with incorrect source activity were delivered to medical institutions since 2017. Specifically, seeds with source activity different from what the institutions had ordered, or dead seeds, were delivered. This has led to a loss of trust in the source activity values published by manufacturers. An investigation conducted in response to these incidents revealed that only 16% of facilities verified source activity, and only 52% verified the number of seeds. Reasons cited for the low verification rates included a lack of necessary equipment, insufficient knowledge of verification procedures, a shortage of staff capable of performing verification, and the fact that many seeds distributed in Japan cannot be re-sterilized. The American Association of Physicists in Medicine (AAPM) advocates for the importance of source strength verification at user facilities. Since we began 125I seed brachytherapy, our facility has continuously performed source strength verification on all delivered blister packs. The single-seed assay recommended by the AAPM is difficult to perform in Japan. As an alternative method, a batch assay using an ionization chamber or a well-type ionization chamber to measure multiple seeds simultaneously has been proposed. At our facility, we calculate the source intensity detection rate from measurements using a well-type ionization chamber to verify the accuracy of the source strength. Our facility uses AgX100 (Buford, GA: Theragenics Corporation) as the source and the CRC-15R well-type ionization chamber (Ramsey, NJ: Capintec Corp.) as the measuring instrument. While there are scattered reports verifying detection rates in batch assays as prior research, there are very few papers discussing regression equations relating the number of sources to detection rates. The objective of this study is to validate a regression equation for predicting appropriate detection rates for each number of radiation sources based on accumulated data. Microsoft Office - Excel 2019 (Redmond, WA: Microsoft Corp.) was used to calculate the regression equation. The guidelines explain how to respond when batch assay results deviate from nominal values. Specific control limits, such as acceptance limits and intervention limits, are indicated. In seed therapy, the insertion of dead seeds or sources with mismatched calibration dates can have significant consequences; therefore, source strength measurement is a critical verification procedure in this treatment modality. We believe the regression equation derived from this study will serve as a useful reference for facilities that have already started 125I seed brachytherapy, as well as those planning to do so in the future.