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.