RESUMO
PURPOSE: Fricke dosimetry has shown great potential in the direct measurement of the absolute absorbed dose for 192Ir sources used in HDR brachytherapy. This work describes the determination of the correction factors necessary to convert the absorbed dose in the Fricke solution to the absorbed dose to water. METHODS: The experimental setup for Fricke irradiation using a 192Ir source was simulated. The holder geometry used for the Fricke solution irradiation was modelled for MC simulation, using the PENELOPE. RESULTS: The values of the factors determined for validation purposes demonstrated differences of less than 0.2% when compared to the published values. Four factors were calculated to correct: the differences in the density of the solution (1.0004 ± 0.0004); the perturbations caused by the holder (0.9989 ± 0.0004); the source anisotropy and the water attenuation effects (1.0327 ± 0.0012); and the distance from the center of the detection volume to the source (7.1932 ± 0.0065). CONCLUSION: Calculated corrections in this work show that the largest correction comes from the inverse squared reduction of the dose due to the point of measurement shift from the reference position of 1 cm. This situation also causes the correction due to volume averaging and attenuation in water to be significant. Future versions of the holder will aim to reduce these effects by having a position of measurement closer to the reference point thus requiring smaller corrections.
Assuntos
Braquiterapia , Radiometria , Simulação por Computador , Radioisótopos de Irídio/uso terapêutico , Método de Monte Carlo , Dosagem RadioterapêuticaRESUMO
This article presents the technical conditions necessary to establish appropriate radiation qualities for the calibration of the dosemeters used in the mammography detectors in the Laboratório de Ciências Radiológicas (LCR) from the Universidade do Estado do Rio de Janeiro. Tests were conducted to evaluate the homogeneity of the radiation field, scattering, half-value layers and system stability. The calibration method (substitution) is described in this work. A moderate alteration in filtration makes it possible to maintain the half-value layers within the limits recommended. The results indicate the adequacy of the LCR laboratory for the calibration of the dosemeters in the radiation qualities for mammography with an expanded uncertainty in the best measurement capability of ± 1.8 % (k = 2).