TG-43 U1 based dosimetric characterization of model 67-6520 Cs-137 brachytherapy source

Purpose: Brachytherapy treatment has been a cornerstone for management of various cancer sites, particularly for the treatment of gynecological malignancies. In low dose rate brachytherapy treatments, ¹³⁷Cs sources have been used for several decades. A new ¹³⁷Cs source design has been introduced (model 67-6520, source B3-561) by Isotope Products Laboratories (IPL) for clinical application. The goal of the present work is to implement the TG-43 U1 protocol in the characterization of the aforementioned ¹³⁷Cs source. Methods: The dosimetric characteristics of the IPL ¹³⁷Cs source are measured using LiF thermoluminescent dosimeters in a Solid Water phantom material and calculated using Monte Carlo simulations with the GEANT4 code in Solid Water and liquid water. The dose rate constant, radial dose function, and two-dimensional anisotropy function of this source model were obtained following the TG-43 U1 recommendations. In addition, the primary and scatter dose separation (PSS) formalism that could be used in convolution/superposition methods to calculate dose distributions around brachytherapy sources in heterogeneous media was studied. Results: The measured and calculated dose rate constants of the IPL ¹³⁷Cs source in Solid Water were found to be 0.930 (±7.3%) and 0.928 (±2.6%) cGy h ^-1 U^-1, respectively. The agreement between these two methods was within our experimental uncertainties. The Monte Carlo calculated value in liquid water of the dose rate constant was =0.948 (±2.6%) cGy h^-1 U^-1. Similarly, the agreement between measured and calculated radial dose functions and the anisotropy functions was found to be within ±5%. In addition, the tabulated data that are required to characterize the source using the PSS formalism were derived. Conclusions: In this article the complete dosimetry of the newly designed ¹³⁷Cs IPL source following the AAPM TG-43 U1 dosimetric protocol and the PSS formalism is provided.