An In-House House Holder for Batch Assaying of the Source Strength of Model STM125 Seed Sources
Abstract
Purpose
It is recommended that a significant number of the seed sources for low-dose-rate permanent brachytherapy (BT) be assayed and their source strengths validated against those stated by the manufacturer before seed implantation. The lack of a requisite dosimeter system and its complementary source holder, combined with concerns about rendering the seed sources unsterile due to their inadequacy, forces the Brachytherapist to rely on the manufacturer’s stated source strengths without validation. Concerning this, there is the possibility of treating patients with dead sources and sources with misrepresented source strengths, which can adversely impact treatment outcomes.
Methods
An in-house source holder, compatible with the Standard Imaging HDR 1000 Plus well-type ionisation chamber (WTIC), was designed and constructed from Perspex (PMMA) sheets. The holder was designed to hold a cartridge of model STM1251 I-125 seed sources within its well. The calibration coefficient of the dosimeter system, including the in-house source holder, was established through cross-calibration with a calibrated WTIC and a commercial source holder for an I-125 seed source borrowed from another institution. To assess the effectiveness and reliability of the in-house source holder, source strength measurements were performed using the fabricated source holder with STM1251 I-125 cartridges containing different numbers of seed sources.
Results
There is a strong linear correlation between the corrected response of the HDR 1000 Plus WTIC and the number of seeds within a cartridge. The intercept of the line of best fit is dependent on the total air kerma strength of a cartridge. The discrepancies between the measured and the manufacturer-stated source strengths range from 0.018 to 2.631% (mean of 1.155 ± 0.854%).
Conclusion
The in-house holder ensured the reproducibility of the setup for source strength measurements. If the holder is sterilised, it can be used to validate the source strength of every seed source before implantation.