Measured transverse‐axis dosimetric parameters of the model STM1251 I125 interstitial source

The recent popularity of permanent implants for treatment of prostate cancer has created a large demand for low energy seed sources. Vendors have introduced new source designs to meet this demand. The AAPM has recommended that all low energy interstitial brachytherapy seed sources be subjected to independent dosimetric evaluations, preferably using experimental measurements as well as Monte Carlo calculations. This work presents the results of Thermo-Luminescence Dosimeter (TLD) measurements of dosimetric parameters on the transverse axis of a new 125I seed source, the Source Tech Medical STM1251 125I seed. Experimental measurements were performed in a Solid Water phantom, with the results corrected to values for liquid water using Monte Carlo calculated correction factors. The parameters measured include the dose rate constant and values of the radial dose function at distances of 0.5 cm through 5 cm. The measured dose rate constant in liquid water for the STM1251 125I seed was 1.039 cGy/U-hr. Measured radial dose function values agreed with Monte Carlo calculated ones to within 10%. These measurements therefore confirm the modeling and simulation of Monte Carlo calculations for this 125I source design, within the statistical uncertainties of the calculation and measurement techniques.