Dosimetric Impact of Shielding In a Co-60 HDR Cylindrical Applicator: A Geant4 Monte Carlo and Gafchromic Film Study
Abstract
Purpose
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. Although the shield thickness in cylindrical applicators was originally designed for Ir-192 sources, the same applicator design is commonly used with Co-60, despite its higher photon energy and the expected differences in transmission. This study evaluated the dosimetric impact of shielding in a Flexitron cylindrical applicator loaded with a Co-60 source and quantified the TG-43 deviations introduced by the high-Z components within the shielded design.
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 Monte Carlo (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
In the presence of high-density shielding, TG-43–based TPS calculations show limited accuracy when a single transmission factor is applied. Advanced approaches, such as Monte Carlo–based or model-based algorithms, are recommended to achieve more accurate dose calculations in the presence of high-Z materials.