Calibration procedures for seeds preloaded in cartridges.

ABSTRACT

Radioactive seeds preloaded in sterilized cartridges or needles are commonly obtainable from manufacturers. Under the US regulations for control of radioactive materials, seed users are required to account for all seeds and independently verify their air kerma strength (SK). As a result, the viability of inspection schemes that rely on measurement of aggregate seeds is of interest. In this paper we consider the conditions (if any) under which cartridge inspection can satisfy regulatory requirements and still provide practical benefit (i.e., time savings) against the regular single-seed assay. The standards for comparison are the recommendations of AAPM TG40, AAPM TG56, and ACR's "Standard for the Performance of Manually Loaded Brachytherapy Sources." The practical benefit is judged in comparison to the effort required to apply the 10% assay recommendation of TG40 to seeds in cartridges. Two specific cartridge inspection schemes are considered (a) measuring the SK of each cartridge in a batch; (b) measuring a single cartridge sampled at random from the batch. Unlike the 10% assay, which is defined (imperfectly, in our view) without reference to the prevalence of in-calibration seeds, the estimation of the relative merits of cartridge inspection methods must necessarily include such information and, as such, is manufacturer specific. In this paper results are provided for Oncura model 6711 125I seeds in shielded and unshielded Mick cartridges. We show that the only practically useful cartridge inspection scheme is the batch scheme applied to unshielded cartridges. The false positive rates associated with the other schemes are such that we expect to open a cartridge (and perform the 10% assay) at least 80% of the time. While anything less than 100% of the time is theoretically an improvement, this neglects the additional effort required to assay the cartridges.