Dosimetric Impact of Shielding in a Co-60 Hdr Cylindrical Applicator: A Geant4 Monte Carlo and Gafchromic Film Study Publisher



Gholami S ; Aloraibi H ; Abtahi M ; Jassim H ; Longo F ; Severgnini M
Source: Radiation Measurements Published:2026

Abstract

Objectives High-dose-rate (HDR) vaginal brachytherapy often employs shielded applicators to enhance organ-at-risk (OAR) sparing—particularly rectal protection—in cases with asymmetric tumor distribution. While cylindrical applicators were originally designed for Ir-192 sources, they are increasingly used with Co-60, despite its higher photon energy and the potential for altered transmission. This study investigates dose calculation deviations for a Co-60 source in a specific shielded cylindrical applicator design to evaluate whether using a single transmission factor within the TG-43 formalism is sufficient in the presence of high-Z shielding, or if a more accurate method is required. Methods Geant4 Monte Carlo (MC) simulations were performed to model Elekta's cylindrical Flexitron applicator in both single-channel and shielded configurations with a Co-60 source. Dose distributions obtained from MC simulations were compared with those calculated by the treatment planning system (TPS). In addition, EBT3 Gafchromic film measurements were performed to provide experimental validation of the dose distributions and to quantify the shield transmission factor. Results For the single-channel applicator, MC, TPS, and film dosimetry were in good agreement, with 95% of points within 3%/3 mm. For the shielded applicator, MC and film showed similar agreement, while TPS calculations progressively deviated, reaching up to 10% at 5 cm from the source. Conclusion Dose evaluation of Co-60 HDR sources using shielded cylindrical applicators highlights TG-43 dosimetric limitations caused by material inhomogeneity. Conventional single-factor shielding corrections are inadequate, underscoring the need for Monte Carlo or model-based algorithms to achieve accurate dose calculations in the presence of high-Z materials. © 2026 Elsevier Ltd.