Current status and issues with the dosimetric assay of iodine-125 seed sources at medical facilities in Japan: a questionnaire-based survey†

In conducting dosimetric assays of seed sources containing iodine-125 (125I), several major guidelines require the medical physicist to verify the source strength before patient treatment. Japanese guidelines do not mandate dosimetric assays at medical facilities, but since 2017, three incidents have occurred in Japan wherein seeds with incorrect strengths were delivered to medical facilities. Therefore, this study aimed to survey the current situation and any barriers to conducting the dosimetric assay of iodine-125 seeds at medical facilities in Japan. We conducted a questionnaire-based survey from December 2020 to April 2021, to examine whether seed assay and verification of the number of seeds delivered were being performed. We found that only 9 facilities (16%) performed seed assay and 28 (52%) verified the number of seeds. None of the facilities used an assay method that ensured traceability. The reasons for not performing an assay were divided into two categories: lack of resources and legal issues. Lack of resources included lack of instruments, lack of knowledge of assay methods, shorthand, or all of the above, whereas legal issues included the inability to resterilize iodine-125 seeds distributed in Japan and/or purchase seeds dedicated to the assay. Dosimetric assays, including simple methods, are effective in detecting calibration date errors and non-radioactive seeds. The study findings suggest that familiarization of medical personnel with these assay methods and investigation of the associated costs of labor and equipment should be recommended, as these measures will lead to medical reimbursement for quality assurance.

[Technical Report: Quality Assurance for 125I Seed Sources in Permanent Prostate Brachytherapy]

This technical report provides useful information on the current status and issues of quality control in ¹²⁵I seed source strength measurement for Permanent Prostate Brachytherapy in Japan.With the spread of ¹²⁵I seed brachytherapy, the traceability of source strength measurements with the single-seed assay was established in Japan. This allows medical facilities to measure reference air kerma rate of ¹²⁵I seeds with their own well-type of ionization chamber. However, it is difficult to maintain the traceability chain because the ¹²⁵I reference air kerma rate standards have been hardly utilized by medical facilities so far. Meanwhile, some serious incidents of contamination of the different source strengths and dead seeds were reported in Japan.To address the specific issues in Japan, JASTRO Brachytherapy Subcommittee established a working group (WG) in 2021. The goal of this WG is to investigate the management methods of source strength measurement used in medical facilities, and to discuss the ideal and practicable methods of source management such as verifying the number of seeds and source strength. Initially, a questionnaire survey was conducted to facilities offering ¹²⁵I seed brachytherapy in Japan. Sixty-seven out of 95 facilities responded (response rate 70.5%). This survey revealed that 41% of facilities did not perform either confirmation of the number of seeds or measurement of source strength. There are several reasons why the source strength was not measured in those facilities. For example, ¹²⁵I seeds are provided under the sterilized conditions; quality assurance by source suppliers is reliable; and there is not sufficient staff.The single-seed assay is regarded as an internationally standardized and the most reliable measurement method. Therefore, it is an essential measurement technique to ensure traceability of source strength measurements. However, our survey found that most Japanese facilities do not perform single-seed assays. Meanwhile, some facilities have performed batch assay as an alternative method, in which all of the multiple sources in a batch are measured while loaded into sterilized cartridges. Although the measurement by the batch assay is less accurate than the one by the single-seeded assay, the batch assay does not require re-sterilization of the source and can be performed quickly. It might be useful to detect unexpected errors such as differences in the number of sources and abnormalities in source strength.In this report, we will introduce several methods of source strength measurement that have been implemented in medical facilities. The quality assurance of ¹²⁵I seed sources in prostate interstitial brachytherapy should be provided not only by the source suppliers but also by the medical facilities that use sources to treat patients. We hope that medical facilities will refer to this technical report and use it as an aid to quality assurance in their own facilities.