Thermoluminescent dosimetry of the Symmetra™ model I25.S06 interstitial brachytherapy seed

Abstract

As the efficacy of brachytherapy prostate treatment is becoming realized, new models of 125I seeds are being introduced. In this article we present thermoluminescent dosimetry (TLD) in a solid water phantom for a new design of 125I seed (UroMed/Bebig Symmetra, Model I25.S06). TLD cubes, LiF TLD-100, from Bicron (Solon, OH) with dimension 1 x 1 x 1 mm3 were irradiated at various distances from the seed at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. The TLD detectors were calibrated by irradiation in a 60Co teletherapy beam. Monte Carlo simulation was used to account for TLD energy dependence and the deviation of solid water composition (as determined by chemical analysis of a sample) from liquid water. Dose rates per unit air kerma strength were determined based on calibrations traceable to the 1999 NIST standard (corrected for NIST measurement errors made in 1999) for the Symmetra seed. Dose data is presented in TG-43 format as a function of distance and angle. Values for lambda, F(r, theta), g(r), and the anisotropy constant are obtained for use in radiation treatment planning (RTP) software. The dose rate constant was determined to be 1.033+/-6.4% cGy h(-1) U(-1), which is comparable to model 6702 and higher than model 6711. We find the relative dose distributions of the Symmetra seed are similar to model 6702, and less anisotropic than model 6711. After accounting for deviation of measured solid water composition from the manufacturer's specification, good agreement between TLD results and Monte-Carlo-aided values was found.

Influence of phantom material and dimensions on experimental dosimetry

“…The importance of the photoelectric effect at these low energies makes dose distributions highly sensitive to phantom composition [5][6][7] and material specific correction factors are required. The higher mean energy of the 192 Ir photon spectrum makes dose distributions around 192 Ir sources less sensitive to phantom composition and Meli et al 8 concluded that for experimental 192 Ir dosimetry, polystyrene and solid water can be considered water equivalent.…”

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

“…Combining these sources of errors in quadrature, we estimate the uncertainties associated with our TLD measurement of the dose rate constant in water for the STM1251 ${}^{125}\mathrm{normalI}$ source to be approximately 7%, not including uncertainties associated with the chemical composition of solid water phantom material and its impact on conversion of dose in solid water to dose in liquid water. As discussed by Patel et al ., 27 experimentally determined calcium content of solid water material may differ from the specified value by as much as 30%, causing additional uncertainties of up to 5% at 1 cm and 25% at 5 cm on the calculated solid‐to‐liquid‐water conversion factors. While our measured dose rate constant of 1.039 cGy/U‐hr for this source differs by approximately 6% from the Monte Carlo calculated value 23 of 0.98 cGy/U‐hr, the difference falls within one standard deviation of the TLD measurement results.…”

“…Since our solid‐to‐liquid water corrections were based upon the nominal Solid Water composition provided by the vendor, the discrepancy between measurement and simulation may be explained, in part, by uncertainties in the calcium content of Solid Water. Patel et al 27 . demonstrated that his Solid Water material contained significantly less calcium than specified by the vendor.…”

NCS Report 20: Dosimetry and quality control of brachytherapy with low-energy photon sources (I-125)

“…A gold rod placed inside the ceramic core, 3.5 mm long and with a 0.17 mm diameter, serves as a radio-opaque marker. The total seed length is 4.56 mm and the external diameter is 0.8 mm [9,10,11].…”