Impact of air gaps on rectal dose in high dose rate intracavitary brachytherapy: Using Monte Carlo simulations

High dose rate intracavitary brachytherapy (HDR-ICBT) is widely used for cervical cancer treatment, delivering high radiation doses to tumor while sparing surrounding organs at risk. However, air gaps between the applicator and the rectal wall can alter dose distributions, thereby affecting treatment outcomes.

This study evaluated the dosimetric impact of air gaps between the HDR-ICBT applicator and rectum using Monte Carlo (MC) simulations and compared the results with Treatment Planning Systems (TPS) calculations.

A custom-designed acrylic phantom replicating HDR-ICBT geometry was equipped with PTW 9112 rectal and PTW 9113 bladder semiconductor probes. Clinical HDR-ICBT treatment plans using a⁶⁰Co source were reproduced. GEANT4 MC simulations were performed to estimate dose deposition at the detector locations with and without an air gap.

Without air gap, TPS calculations and MC simulations showed good agreement, with mean percentage deviations ranging from 1.02% to 2.08%. With air gap, rectal detector positions showed systematic dose increases of 5-6%, whereas bladder detector position showed a smaller deviation of 3.16%. Across rectal detector positions, deviations ranged from -6.32% to 4.96% without air gap and from -1.94% to 11.52% with air gap. Box plot analysis showed a positive shift and increased variability under the air-gap condition.

Air gaps between the applicator and rectum introduce significant spatially dependent dose perturbations during HDR-ICBT, which are not accounted for in conventional TPS calculations. These findings highlight the importance of tissue heterogeneities in brachytherapy and MC-based dose verification to improve treatment accuracy and patient safety.