Calibration Seed Sampling for Iodine-125 Prostate Brachytherapy
Use of iodine-125 seeds for intraoperative planning and delivery of low dose rate brachytherapy treatments for prostate cancer requires that the air kerma strength of the seeds be checked against the vendor-supplied calibration certificate at the beginning of the surgical procedure. In practise, activity checks of multiple sources are difficult to achieve in surgery. This study therefore investigated the reliability of a calibration method that sampled only one seed, or a small number of seeds, by evaluating the consistency the air kerma strengths of all seeds in three small batches of 10, 20 and 30 iodine-125 seeds and calculating the probability of achieving results representative of each batch, within different levels of uncertainty. For the cartridges containing 10, 20 and 30 seeds, the mean differences between the source strengths identified by physical measurement and their decay-corrected calibration certificate values were respectively \( 5.0\% \pm 4.4\% \), \( - 2.7\% \pm 3.7\% \), and \( 0.4\% \pm 2.8\% \). Assays of 5 randomly sampled seeds were shown to produce results within 3% of the mean air kerma strength of each batch, with larger assays producing less uncertainty. For these seeds, there was a greater than 30% chance that a randomly selected seed would have an activity that differed by more than 3% from the mean activity of all seeds in the cartridge. Although attractive as an efficiency measure, the testing of just one seed from a cartridge of iodine-125 seeds has a significant probability of producing an activity measurement that is not representative of the activity of the other seeds in the cartridge, potentially leading to substantial inaccuracies in implant dosimetry.
KeywordsRadiation therapy Brachytherapy Quality assurance
This work was made possible by the thoughtful ordering and preservation of seed cartridges that was provided by Candice Deans, Mark West and George Warr.
Compliance with Ethical Standards
The authors declare that they have no conflict of interest. This article does not contain any studies with human participants or animals performed by any of the authors.
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