Simulación Monte Carlo para determinar la dosis absorbida en agua de referencia con calidad de radiación de Co-60 utilizada para la calibración de sistemas dosimétricos en el Laboratorio Secundario de Calibración Dosimétrica en Colombia

Abstract

The Secondary Standard Dosimetry Laboratory (SSDL) of the Colombian Geological Service (SGC) provides calibration services for dosimetric systems used in external beam radiotherapy. The calibration method is based on the Technical Reports Series No. 469, which describes the experimental setup consisting of a reference beam produced by a Hopewell Designs irradiation system containing a Co-60 radioactive source and a water phantom. The purpose of the calibration is to ensure the traceability of dosimetric measurements to the International System of Units. To characterize the national reference radiation beam and optimize the accuracy in determining the absorbed dose rate in water (Ḋw), the Monte Carlo method is employed using TOPAS MC. A detailed simulation of the technical and reference conditions present at SSDL is performed, analyzing beam profiles and percentage depth dose with a Farmer-type ionization chamber to assess the homogeneity of the irradiation field. The simulation results show that Compton scattering is the predominant interaction mechanism, validating the dosimetric system configuration. Additionally, the obtained uncertainties are approximately 3%, close to the tolerance limit established by Venselaara, Welleweerdb, and Mijnheerc. The qualitative comparison between the simulated and experimental beam profiles and PDD curves supports the accuracy of the Monte Carlo method in characterizing the reference beam at SSDL. These preliminary findings suggest that the simulation is on the right track and provide a solid basis for improving dosimetric calibration procedures.