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

Background and purpose: 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. Materials and methods: 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. Results: 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. Conclusion: 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.