Determination of dosimetric parameters for shielded ¹⁵³Gd source in prostate cancer brachytherapy

Interstitial rotating shield brachytherapy (I-RSBT) is a recently developed method for treatment of prostate cancer. In the present study TG-43 dosimetric parameters of a ¹⁵³Gd source were obtained for use in I-RSBT. A ¹⁵³Gd source located inside a needle including a Pt shield and an aluminum window was simulated using MCNPX Monte Carlo code. Dosimetric parameters of this source model, including air kerma strength, dose rate constant, radial dose function and 2D anisotropy function, with and without the shields were calculated according to the TG-43 report. The air kerma strength was found to be 6.71 U for the non-shielded source with 1 GBq activity. This value was found to be 0.04 U and 6.19 U for the Pt shield and Al window cases, respectively. Dose rate constant for the non-shielded source was found to be 1.20 cGy/(hU). However, for a shielded source with Pt and aluminum window, dose rate constants were found to be 0.07 cGy/(hU) and 0.96 cGy/(hU), on the shielded and window sides, respectively. The values of radial dose function and anisotropy function were tabulated for these sources. Additionally, isodose curves were drawn for sources with and without shield, in order to evaluate the effect of shield on dose distribution. Existence of the Pt shield may greatly reduce the dose to organs at risk and normal tissues which are located toward the shielded side. The calculated air kerma strength, dose rate constant, radial dose function and 2D anisotropy function data for the ¹⁵³Gd source for the non-shielded and the shielded sources can be used in the treatment planning system (TPS).