Early tumor shrinkage correlates with final EBRT response in cervical cancer patients treated with MRI-guided radiotherapy

Background/Purpose: Tumor response can vary significantly in locally advanced cervical cancer patients undergoing radiotherapy. Personalizing treatment based on this variability may improve therapeutic outcomes. This study quantified daily tumor volume evolution on MR-Linac and explored its potential in predicting treatment outcome. Materials/Methods: Sixteen patients were retrospectively analyzed. Daily T2-weighted MRI scans (n = 400) were used to delineate tumor volumes, validated by two radiation oncologists. Tumor volume reduction rates (TVRR) and regression slopes were assessed across fractions. Spearman analysis evaluated correlations with final external beam radiotherapy (EBRT) response. To assess the impact of treatment interruptions, regression slopes were compared between consecutive (1-day interval) and interrupted delivery (>1 day) across early, middle, and late phases. Results: Median tumor volume decreased from 26.2 cc at baseline to 0.7 cc at fraction 25. TVRR showed progressively stronger associations with end-of-EBRT response, with Spearman coefficients increasing from rho = 0.20 (p = 0.45) at fraction 5 to a peak of rho = 0.67 (p = 0.004) at fraction 10. All patients achieving > 50% reduction by fraction 10 reached > 90% reduction by the end of EBRT. Treatment interruptions were associated with significantly slower regression kinetics, particularly during the early phase (p = 0.003). Conclusions: Early tumor reduction by fraction 10 is strongly associated with final EBRT response, supporting its role as a potential predictive marker for adaptive strategies. Furthermore, treatment continuity appears critical for optimal regression kinetics in the initial phase. Validation in larger cohorts is needed.