KEY POINTS
- TOPAS Monte Carlo simulations modeled modular superficial brachytherapy applicators loaded with three beta sources—P-32, Sr/Y-90 and Ho-166—and three photon sources—Pd-103, I-125 and Yb-169. Circular and hexagonal geometries, planar and curved surfaces, simultaneous versus sequential placement, and active radii of 5.50 versus 2.75 mm were evaluated.
- Source choice produced markedly different depth-dose behavior. At 6 mm depth, normalized beta dose was only 0.04% for P-32, 0.12% for Ho-166 and 1.40% for Sr/Y-90; at 40 mm, photon dose remained 0.54% for Pd-103, 1.34% for I-125 and 2.27% for Yb-169.
- Simultaneous beta-source placement produced substantial peak-to-valley modulation because inactive cladding created gaps between adjacent active regions. For hexagonal placement, PVR was 11.6 with P-32, 7.3 with Sr/Y-90 and 10.1 with Ho-166, versus 25.3, 14.1 and 22.0 with circular arrangements.
- Sequentially shifting applicators over multiple positions substantially flattened the dose distribution. Beta-source flatness with hexagonal sequential delivery was only 2.1% for P-32, 1.2% for Sr/Y-90 and 1.9% for Ho-166, outperforming the corresponding circular configurations.
- Applicator radius strongly influenced beta-source uniformity. Reducing radius from 5.50 to 2.75 mm lowered circular PVR from 13 to 2.2 for P-32, 7.7 to 1.6 for Sr/Y-90, and 13.8 to 2.0 for Ho-166. Photon sources were less geometry-sensitive, with PVR already around 1–3.
- On curved surfaces, smaller beta-source modules increased feasible placement positions by 57–89%, increased mean dose at 1-mm depth by 28–58%, and reduced PVR by 33–64%, demonstrating the benefit of denser sequential coverage.
- These are purely computational results. Practical implementation still requires physical prototype construction and testing of placement accuracy, immobilization, treatment duration, source inventory, shielding, sterilization, quality assurance and reproducibility before clinical use.
CLINICAL TAKEAWAY
For modular superficial brachytherapy on irregular anatomy, smaller applicators and sequential repositioning may be more important than applicator shape itself, particularly with beta emitters. The work is useful for device development but remains an engineering simulation rather than evidence for a deployable clinical system.