Dosimetric Comparison of Cervix Boost RT with CBCT-Based Adaptive Versus Non-Adaptive Instead of Brachy Boost
Abstract
Purpose
Brachytherapy (BT) remains the standard modality for dose escalation in cervical cancer boost targets, with well-established high-dose-rate (HDR) EQD2 constraints based on HR-CTV D90 and organ-at-risk (OAR) D2cc metrics. However, BT is not feasible for all patients. This study evaluates ability of online adaptive external-beam radiotherapy (ART) to achieve BT-derived EQD2 constraints for target coverage and OAR sparing and compares its performance with a non-adaptive EBRT boost.
Methods
We conducted a single-institution retrospective dosimetric analysis of cervical cancer patients treated with EBRT boost using ART on the Varian EthosTM. Boost prescriptions were 5–6Gy×5 fractions, with maximum dose limited to 120–130% of prescription, markedly lower than >200% hotspots of BT. For each fraction, ART plans were compared with non-adaptive plans recalculated on daily anatomy. HR-CTV-D90 and bladder and rectum D2cc and D0.03cc were evaluated per fraction and accumulated with pelvic EBRT, with all doses converted to EQD2 (α/β=10Gy for tumor,3Gy for OARs).
Results
Comparing EBRT boost strategies, ART achieved higher and more consistent coverage than non-adaptive, with a higher median CTV-D90 (7.89Gy vs 6.75Gy). OAR dose metrics were similarly improved with bladder D2cc and D0.03cc lower with ART (median D2cc:4.30Gy vs 4.61Gy; D0.03cc:5.41Gy vs 5.95Gy). Rectum dose metrics showed same trend, with lower median D2cc (2.9Gy vs 3.6Gy) and D0.03cc (3.9Gy vs 5.1Gy). In cumulative EQD2 analysis, ART increased mean CTV-D90 (85.6 vs 81.6Gy EQD2) and improved coverage margins (+0.6 vs –3.4Gy), while also improving bladder (11.5 vs 7.6Gy) and rectum D2cc margins (5.4 vs –0.9Gy). Paired comparisons demonstrated statistically significant differences across all evaluated metrics (p≤0.002).
Conclusion
Adaptive EBRT boost mitigates anatomy-driven dosimetric variability, enabling delivery of BT-derived EQD2 thresholds for target coverage and OAR sparing. While EBRT boost cannot reproduce dose heterogeneity of BT, ART provides a clinically meaningful alternative for non-BT candidates, representing advancement over non-adaptive approaches.