Dose modeling of superficial brachytherapy applicator geometries using multiple radioisotopes

BACKGROUND: Superficial brachytherapy enables localized dose delivery for superficial cutaneous lesions, but conventional applicators may be limited in treating extended, irregular, or curved surfaces. Geometric constraints, gaps or overlaps between adjacent applicators, and placement-related uncertainties can reduce dose uniformity, underscoring the need to optimize applicator geometry and deployment strategy. PURPOSE: To enhance the efficacy of superficial brachytherapy for irregular or curved cutaneous surfaces, we conducted a dosimetric analysis of beta and photon sources utilizing modular circular and hexagonal applicator geometries. The study emphasizes dose uniformity, the impact of applicator dimensional variations, and the evaluation of alternative deployment strategies. METHODS: Monte Carlo simulations were performed for beta sources (P-32, Sr/Y-90, and Ho-166) and photon sources (I-125, Pd-103, and Yb-169). Applicator models incorporating a zirconia absorber with ceramic and tungsten shielding were constructed, and dose distributions were scored in a water-equivalent medium. Percent depth-dose (PDD) curves were generated for beta sources, and photon sources. Multi-applicator configurations were evaluated on planar and curved surfaces under both simultaneous and sequential delivery. Applicator-size effects were examined using two active radii (5.50 mm and 2.75 mm). Dose uniformity was quantified using the mean dose and the peak-to-valley ratio (PVR) at 1 mm depth. RESULTS: Beta sources were strongly affected by applicator size and geometry, producing larger variations in near-surface dose uniformity than photon sources. Sequential delivery reduced peak-to-valley modulation for all radioisotopes. Reducing the applicator radius increased the number of deliverable positions on curved surfaces by 60%-83%, increased mean dose by 28%-71%, and decreased PVR by 33%-86%, indicating improved uniformity. Photon sources showed more stable behavior, with lower sensitivity to applicator geometry and size. CONCLUSION: Radioisotope type, applicator radius, and delivery mode significantly influence superficial dose distributions. Smaller applicators and sequential deployment improve dose uniformity, particularly for beta sources. These results provide practical guidance for radioisotope selection and modular applicator design in superficial brachytherapy.