Purpose: Ophthalmic applicators used in brachytherapy are placed in contact or close to the region to be treated, administering a high dose gradient in a few millimeters, making the determination of dosimetric parameters become a challenge. Thus, the TG-43 protocol proposed by the American Association of Physicists in Medicine (AAPM) recommends the use of Monte Carlo simulation to determine the dosimetric parameters of this type of source. Therefore, the objective of this work was to analyze the relative dose distributions through the TOPAS simulation code from a source ⁹⁰Sr/⁹⁰Y used in brachytherapy. The obtained results were compared with the PENELOPE code.
Methods: The applicator simulated in this work was the model SIA.20, which contains a source of ⁹⁰Sr/⁹⁰Y, with 9 mm in diameter used in the treatment of ophthalmic brachytherapy. A spherical phantom with 30 mm in diameter, representing the patient's eye, was placed below the applicator. For the evaluation of the TOPAS code, a beam profile was obtained through a dose distribution at 1 mm depth of the phantom, as recommended by dosimetry protocol. The shape of the beam profile is similar to a Gaussian curve, so, the values of average width at half maximum (FWHM) were compared with the PENELOPE code, as the same condition of simulations.
Results: The FWHM values were 0.219 and 0.362 for PENELOPE and TOPAS, respectively. Comparing the values of dose relative at 2.5 mm from the center of the source, a region with a high dose gradient, a difference of 5.5% was found, due to the contribution of the electron edge effects.
Conclusion: The results obtained in this work show that the TOPAS code presents as a promising tool for dosimetry in brachytherapy.