# Impact of rectal gas evacuation on dosimetry and applicator displacement in cervical cancer brachytherapy

## TL;DR

Evacuating rectal gas during cervical cancer brachytherapy reduces rectal dose but may shift the applicator, requiring careful monitoring.

## Contribution

This study quantifies the dosimetric and applicator displacement effects of rectal gas evacuation in cervical cancer brachytherapy.

## Key findings

- Rectal volume decreased by 40.1% after gas evacuation, significantly reducing rectal radiation doses.
- Applicator tip and cervical stopper displacements averaged 5.86 mm and 4.23 mm, respectively.
- Bladder volume increased by 18.2%, but other OARs showed no significant dose changes.

## Abstract

This study aimed to evaluate the impact of rectal gas evacuation on organ‐at‐risk (OAR) volumes, dose‐volume histogram (DVH) parameters, and applicator displacement during cervical cancer brachytherapy.

Twenty‐one cervical cancer patients who received three‐dimensional brachytherapy at our center between November and December 2024 and presented with rectal gas were retrospectively included. Planning computed tomography (CT) images were acquired before and after rectal gas evacuation to evaluate changes in rectal and bladder volumes, as well as radiation dose variations to OARs (bladder, rectum, sigmoid, and small intestine). Dosimetric parameters analyzed comprised D
0.1cc, D
1cc, D
2cc, and D
5cc (minimum doses delivered to the most irradiated 0.1, 1, 2, and 5 cm3 of the OARs, respectively), as well as D
max (maximum dose) and D
mean (mean dose). Displacements of the applicator tip and cervical stopper were quantified using a coordinate system based on pelvic bony landmarks.

Rectal volume decreased by 40.1% after gas evacuation, while bladder volume increased by 18.2%. D
0.1cc, D
1cc, D
2cc, D
5cc, and D
max in the rectum decreased significantly (P < 0.001) after gas evacuation, whereas no significant changes were observed in the DVH parameters of the other OARs. The mean displacements of the applicator tip and cervical stopper were 5.86 ± 3.64 mm and 4.23 ± 3.30 mm, respectively.

Rectal gas evacuation results in a statistically significant and clinically relevant reduction in rectal volume and rectal dose, underscoring its importance as a routine clinical procedure. However, as it may induce millimeter‐level applicator displacement with clinically measurable dosimetric consequences, careful monitoring is warranted, with post‐evacuation replanning or, if necessary, applicator adjustment.