Objective: In the majority of commercially available treatment planning systems used for high-dose rate brachytherapy have TG-43 formalism for dose calculation. This formalism overlooks the heterogeneities present in the patient for dose calculation. The current study used Gafchromic EBT3 film dosimetry and Monte Carlo simulations to evaluate their impact on the dose received by the rectum during HDR brachytherapy of the cervix.

Materials and Methods: In this study, a straight stainless-steel central tandem with a 35 mm polyetheretherketone vaginal cylinder was placed in a cylindrical phantom made of Elasto-gel. In this cylindrical phantom, EBT3 Gafchromic films were placed for dose measurements at 1.9 cm from the central tandem. The results were ascertained using Monte Carlo simulations.

Results: The treatment planning systems overestimated the dose for the straight central metallic tandem when used with a 35 mm vaginal cylinder. The overestimation of dose was all along the vertical axis of the applicator at 1.9 cm from the central tandem. The maximum overestimation dose values obtained using Gafchromic EBT3 film dosimetry and Monte Carlo simulations were 6.36% and 6.18%, respectively.

Conclusions: The difference between the practical dose and the dose calculated by the treatment planning system should be considered for the proper estimation of the dose to the rectum. These findings suggest that cervical applicator effects should be considered for better rectal dose estimation.