Dados do Trabalho

Título

Evaluation of dosimetric parameters in an ophthalmic brachytherapy source, obtained in water and perspex, using Monte Carlo simulation

Introdução

Dosimetry in brachytherapy is important to ensure equality between the planned dose and that administered to the patient. However, experimental determination is complex due to the high dose gradient in regions close to the source. In this sense, the American Association of Physicists in Medicine (AAPM) proposed a formalism for calculating the dose using dosimetric parameters, recommending the use of Monte Carlo simulation to calculate these parameters.

Objetivo

In this work, dosimetric parameters were determined: anisotropy function and radial dose function, including relative dose profiles, for an ophthalmic brachytherapy source, in two materials, water and perspex, using Monte Carlo simulation with PENELOPE package.

Método

The brachytherapy source, model IR06-103Pd, commonly used in ophthalmic treatments, was modeled in the center of a cubic phantom with 30 cm of side, filled with water and perspex, in different simulations. The cut-off energy of photons, electrons and positrons was 5 keV, using the 8 main photons emission lines; the number of primary particles remained constant at 5x10⁸, and the history condensation parameters were 0.3 and voxel size was 0.3 cm³ in all simulations.

Resultados

Comparing the relative doses, the greatest uncertainty was found to be less than 4% for regions with a high dose gradient, and the greatest difference found was approximately 5% greater for perspex, at 6 cm from the source. The anisotropy function was calculated at the reference distance in brachytherapy (1 cm) and compared between the analyzed materials.. The highest relative uncertainty found in this dosimetric parameter was approximately 2.2%, when the phantom was filled with water, below the maximum relative uncertainty value recommended by the AAPM. When the results were compared, differences up to 0.03 (absolute value) in the anisotropy function were found from 0º to 40º and from 140º to 180º. And for the radial dose function, the maximum uncertainty found was 0.97%, for the reference distance in brachytherapy, noting that perspex presented a difference 10% greater than water, 1.5 cm from the source.

Conclusão

The results of this work show that the PENELOPE package as a promising tool in the dosimetric calculation of brachytherapy and the importance of choosing the material on which the dosimetric parameter is calculated, in order to minimize errors in the dosimetric calculations of this type of source.

Palavras-chave

Dosimetry, brachytherapy, PENELOPE

Área

Radioterapia