# Dosimetric comparison between brachytherapy and MR-Linac as a boost modality for locally advanced cervical cancer

## TL;DR

This study compares brachytherapy and MR-Linac as boost treatments for cervical cancer, finding differences in dosimetric outcomes and organ exposure.

## Contribution

The study introduces a novel intra-patient comparison of brachytherapy and MR-Linac for cervical cancer boost treatment, highlighting dosimetric differences.

## Key findings

- Brachytherapy delivers higher dose-volumes to targets compared to MR-Linac.
- MR-Linac increases the distance between the tumor and organs at risk when an applicator is in place.
- Rectum doses are higher with MR-Linac, suggesting potential risks for this modality.

## Abstract

•BT and MRL treatment plans were compared intra-patient, within two patient groups.•The dosimetric performance of MRL with respect to BT differed between groups.•High dose-volumes are larger for BT, low dose-volumes are larger for MRL.•Distance of CTVHR to rectum, sigmoid and bowel surface increases with applicator in-situ.•The influence of MRL dose distributions on clinical outcomes must be investigated.

BT and MRL treatment plans were compared intra-patient, within two patient groups.

The dosimetric performance of MRL with respect to BT differed between groups.

High dose-volumes are larger for BT, low dose-volumes are larger for MRL.

Distance of CTVHR to rectum, sigmoid and bowel surface increases with applicator in-situ.

The influence of MRL dose distributions on clinical outcomes must be investigated.

Standard treatment for locally advanced cervical cancer (LACC) is chemoradiotherapy followed by a brachytherapy (BT) boost. However, BT is not always feasible and magnetic resonance (MR)-guided adaptive radiotherapy on the MR-Linac (MRL) might be an alternative. To investigate the dosimetric feasibility of MRL, BT and MRL treatment plans were compared intra-patient in terms of dosimetric differences, next to anatomical and conformity variations.

Two groups of ten patients with LACC treated with BT boost were selected: group 1 included patients for which at least one clinically established (EMBRACE II) treatment planning constraint was not achieved during BT, in group 2 all planning constraints were achieved.

BT treatment plans were compared with MRL treatment plans (based on MRI scans without applicator in place) intra-patient, in terms of dose-volume histogram (DVH) parameters, target-to-OAR (organ at risk) surface distances and conformity ratios.

Group 1 resulted in similar prescribed target dose levels for MRL compared to BT, for group 2 all prescribed target dose levels were significantly higher for BT. Rectum D2cm3 was higher for all MRL treatment plans. Volumes of higher dose levels were larger for BT, volumes of lower dose levels were larger for MRL and the CTVHR to OAR (rectum, sigmoid, bowel) surface distance was greater for BT.

This retrospective study demonstrates that with an MRL boost plan, in some situations it is possible to achieve established planning constraints. However, as rectum doses are higher and dose distributions are fundamentally different, BT remains the modality of choice. Clinical trials are necessary to investigate the influence of the MRL dose distribution on oncological outcomes.