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Session: Adaptive Treatment Planning and Delivery [Return to Session]

Dosimetric Evaluation of a CBCT-Based Daily Adaptive Radiotherapy Protocol for Locally Advanced Cervical Cancer

D Branco*, J Mayadev, K Moore, X Ray, UC San Diego, La Jolla, CA


SU-H300-IePD-F6-1 (Sunday, 7/10/2022) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 6

Purpose: Evaluate the dosimetric improvements to CTV and OARs for a CBCT-based daily adaptive workflow in cervical cancer.

Methods: Using a Varian EthosTM emulator and KV-CBCT scans, we simulated adaptive sessions for 12 retrospective patients. The impact of adaptation on target dose was evaluated by comparing CTV D99%(%) for non-adapted standard-of-care (SOC) plans (5-15mm CTV-to-PTV margins) versus adapted plans (3mm margins). OAR sparing was compared for bowel (V45Gy(cc), V40Gy(cc)), bladder (D50%(cGy)), and rectum (D50%(cGy)). Auto-segmented bowel (Ethos bowel) required considerable contouring edits. Corrections during each adaptive fraction were made to the Ethos bowel only when edits were needed near the CTV. Remaining edits were considered irrelevant for plan optimization as they were far from the target but were performed afterwards for accurate dose metrics for comparison. Volume differences between the Ethos and true bowels were recorded.

Results: CTV D99% values for adapted plans were comparable to non-adapted plans (average difference of -0.9%), while all OAR metrics improved with adaptation. Specifically, bowel V45Gy and V40Gy decreased on average by 87.6cc and 109.4cc, while bladder and rectum D50% decreased by 37.7% and 35.8% respectively. The auto-segmented Ethos bowel was consistently smaller in volume than the true bowel (57/60 fractions) by an average of 73.8cc±102.2cc. Unadapted plans were on average 110.6cc and 175.1cc for V45Gy and V40Gy, respectively. For the adapted plans, V45Gy and V40Gy were 23.0cc and 65.7cc, respectively.

Conclusion: All OAR dose metrics improved with daily adaption and could translate to reduced toxicity while maintaining tumor control. The auto-segmented bowels consistently had smaller volumes when compared to the true bowel. However, the true bowel with adaptation had substantially decreased V45Gy values compared to non-adapted true bowel values thus demonstrating the dosimetric benefit of adaptation despite the practical limitation of allowing bowel autosegmentation errors to remain uncorrected in the daily adapted workflow.

Funding Support, Disclosures, and Conflict of Interest: Kevin COI: Kevin Moore acknowledges consulting fees and honoraria from Varian Medical Systems. Jyoti COI: Honoraria for lectures and presentations for Varian Xenia COI: Xenia Ray acknowledges Honoria and speaker fees from Varian Medical Systems and a research agreement with Varian Medical Systems


Cone-beam CT, Image-guided Therapy, Dosimetry


TH- External Beam- Photons: adaptive therapy

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