Design and Optimization of an HDR 192Ir Brachytherapy Applicator for Skin Irradiation Using Monte Carlo-based Dosimetry

Abstract

Purpose/Objective(s): The round applicators are designed to deliver boost dose to the breast in a non-invasive manner instead of using an electron boost. By augmenting the round applicator design through adding an internal truncated cone, the conical applicators are able to concurrently reduce treatment time and dose to OARs such as the skin. By adding an internal wedge to the conical applicators, skin cancers may also be treated. The purpose of this study was to design an applicator that could deliver brachytherapy to the skin in a conformal and homogeneous manner. Materials/Methods: The 4 cm, 6 cm, and 8 cm diameter round applicators served as the foundation for variations of internal cone, internal wedge, and applicator height. These three parameters were varied to irradiate 1 cm, 2 cm, and 3 cm diameter targets. Using Monte Carlo (MC) methods with the MCNP5 radiation transport code, the 4116, 6116, 6216, 6316, 8216, 8316, 4125, 6125, 6225, 6325, 8225, and 8325 designs were evaluated, where the first digit indicates applicator diameter (cm), the second digit is target diameter (cm), and the last two digits are applicator height (either 16 mm or 25 mm). Dose distributions were calculated in a soft tissue cubic (16 cm) phantom with results assessed using (0.5 mm) voxels. The prescription dose (DRx) was normalized to 100% at a depth d Z 3 mm on the central axis. Typical statistical uncertainties (k Z 1) were < 0.4%. The applicator design goal was to provide a homogeneous dose distribution for tissue 3 mm from the surface while protecting tissue at d 5 mm. Quantitative comparisons were made to the dose distributions produced by the 2 cm (VH2) and 3 cm (VH3) diameter Valencia skin brachytherapy applicators. Results: Among the twelve designs studied, 8216 had the best dose coverage and conformity. It covered a 2 cm target on the skin surface like the VH2. The 8216 depth dose dropped more dramatically for d 5 mm than for the VH2 applicator. Doses at 10 mm and 20 mm deep on the central axis were 15% and 14% of DRx for design 8216, and 52% and 27% for VH2, respectively. Maximum skin dose for design 8216 was 113%, and 139% for the VH2. Dose was 7x larger for design 8216 on the skin surface several centimeters beyond the field edge due to less lateral attenuation than the VH2 design. However, dose was still <35% of DRx. Conclusion: The round skin applicator designs examined in this study provided better depth conformity with less lateral conformity. However, at least one round skin applicator provided comparable dose distribution as the Valencia skin applicators. Other potential skin applicator designs are being investigated and clinical evaluation within a 3D treatment planning system is underway. Current results suggest that the round skin applicator approach may be amenable, if not favorable, for the treatment of larger skin lesions. Author Disclosure: Y. Yang: None. M.J. Rivard: K. Stock; Dr. Rivard is a shareholder of Advanced Radiation Therapy (ART) LLC.