Design and construction of in-house source holder for assaying the source strength of I-125 seed, model #: STM1251

Introduction: Assessment of the source strength is a mandatory task performed by medical physicists to ensure accurate, optimal dose calculation and delivery to a patient undergoing brachytherapy (BT) treatment. Furthermore, there is the need to verify source strength provided by a vendor of BT sources before their clinical use to ensure traceability to international standards such as the American Association of Physicists in Medicine and European Society for Therapeutic Radiology and Oncology (AAPM-ESTRO) recommendations. Material and methods: An in-house source holder that could be accommodated within the well-type ionisation chamber (WTIC) was designed and constructed from Perspex (PMMA). The constructed source holder was designed to hold a cartridge of I-125 BT seed within the WTIC to facilitate batch source strength measurement. The source holder ensuring reproducibility of I-125 seed placements during source calibration, also enabled placement of a cartridge such that the point of concentration of activity of I-125 seeds coincided with the calibration point. Results: The mean percentage deviation of the source strengths was (1.155 ± 0.854) % which agrees with AAPM-ESTRO recommendations. For all batches of I-125 seeds investigated in this work, we found that the WTIC response was linear. Conclusions: We, therefore, recommend that the methodology adopted to assay the source strength of Bard I-125 seed (model #: STM1251) be used by the BT departments offering low dose rate permanent interstitial BT services.