Brachytherapy of skin cancer using a ⁹⁰Sr/⁹⁰Y beta-emitting source: design of a fixed geometry applicator

Beta radiation is a suitable candidate for treatment of skin cancer due to its rapid dose fall-off and better protection of normal tissues. This study aimed to design and propose a set of new fixed geometry applicators with a ⁹⁰Sr/⁹⁰Y beta source for treatment of superficial skin tumors.

First, the ⁹⁰Sr/⁹⁰Y source of the Beta-Cath™ system supplied by Novoste Corporation was simulated with the MCNPX Monte Carlo code and validated using the proposed Task Group No. 60 dosimetric parameters. Then, considering parameters including the source-to-skin distance, treatment time, the shape and thickness of the flattening filter, the size and composition of the sources, and the dimensions of the field, the optimal applicator was selected in terms of flatness, symmetry, penumbra, leakage, and dose rate at different depths (1.0-3.0 mm).

The mean difference in dose rate at the reference point, radial dose function, anisotropy function and two-dimensional dose rate compared to the corresponding published values was less than 3.0%. For a treatment depth of 3.0 mm, treatment times ranged from 6.97 to 10.22 min, the flatness values ​​of the optimal applicators in different sizes ranged from 3.36% to 14.94%, symmetry from 0.3% to 1.7%, and penumbra from 2.7 to 3.6 mm. Leakage within 1.0 mm of the applicator wall was less than 1% of the skin surface dose.

This is the first study to evaluate the design of a fixed geometry applicator for a ⁹⁰Sr/⁹⁰Y beta-emitting source for skin cancer treatment. The designed applicator exhibited good performance in terms of dose distribution parameters such as flatness, symmetry, penumbra, and leakage.