Ananya Choudhury

Quantifying the dosimetric impact of online daily adaptation for MR-guided RT in cervical cancer

Purpose: This study assessed the inter- and intra-fractional dosimetric impact of MR-Linac-based adaptive radiotherapy for cervical cancer (CC). Methods: A retrospective analysis of five node-negative, locally advanced cervical cancer patients treated under the MOMENTUM study (NCT04075305) using adapt-to-shape (ATS) on an Elekta Unity MR-Linac. Assessing the dosimetric impact of daily online adaptations: (1) comparing dose between daily adapted (MR-adapted) and non-adapted (MR-guided) plans, by quantifying dose differences relative to reference plans (by 2 and 5%) and evaluating adaptation frequency; (2) performing intra-fraction dose evaluations. Dose metrics for targets and organs at risk (OARs) were evaluated following EMBRACE II guidelines. Results: MR-adapted plans improved target coverage or reduced OAR dose in 82–100% of fractions at a 2% dose deviation (and in 25–84% at a 5% deviation), compared to MR-guided plans. Dose reductions for OARs ranged from 2 to 8% for D0.1%, 4.77–16.70% for V4000cGy and 2.10–14.00% for V3000cGy. Intra-fraction analysis showed that the difference between daily planned and delivered doses in all target structures was not clinically significant, ranging from 0.08 to 2.20%, except two fractions that experienced higher deviations (5%) in ITV45. Treatment was well-tolerated, with no Grade 2 or 3 toxicities reported. Interpretation: MR-guided plans required adaptation in 25–100% of the fractions when compared to MR-adapted plans. MR-adapted plans demonstrated enhanced target dose consistency and reduced OAR dose for all patients, highlighting the benefits of daily adaptation. Despite longer treatment times, dose accuracy was preserved. Toxicity results for MRgART in CC appear promising.