Dosimetric Analysis for a Modern 131 Cs Brachytherapy Seed
Abstract
Purpose
Conventional LDR brachytherapy seeds rely on internal radiopaque markers that were incorporated to enhance radiographic visibility. With widespread adoption of modern 3D imaging modalities such as CT, the marker presence degrades seed localization and tissue identification. This work focuses on redesigning the 131Cs seed by retaining the external geometry while eliminating the radiopaque marker.
Methods
The MCNP6 radiation transport code was used to model multiple 131Cs seed designs: CS-1 Rev2 (baseline model), with Ar replacing the Au marker, and with the ceramic core replacing the Au marker. Simulations of 2x109 histories for the three seeds in water and vacuum permitted evaluation of the TG-43 brachytherapy dosimetry parameters, and comparison of baseline results to AAPM TG-43U1S2 reference dosimetry. The three designs were compared to assess the impact of internal structure on dose distribution.
Results
The dose distributions and dosimetry parameters agreed with the reference consensus data within 0.1%. The radial dose function and dose-rate constant changed <1% for the three designs. Removal of the Au marker resulted in 18% higher photon emissions and as a result, less 131Cs is needed for the same absorbed dose. Removal of the Au marker also resulted in more uniform dose distributions with the 2D anisotropy function having larger values than the baseline. This was especially pronounced along the seed long-axis with 10% changes observed at small radii (r≤1cm) and low polar angles (θ≤10°).
Conclusion
The baseline results were benchmarked against reference data and demonstrated excellent dosimetric agreement, which bolstered confidence for evaluating the new seed designs. Both new seed designs without Au markers were dosimetrically similar to the baseline, but with improved dosimetric isotropy. The new seed designs may be used clinically and should be compatible with modern imaging over planar radiographic techniques.