Daily CBCT-Guided Online Adaptive Radiotherapy for Enhanced Personalization and Precision In the Treatment of Muscle Invasive Bladder Cancer
Abstract
Purpose
Standard image-guided radiotherapy(IGRT) approaches for muscle invasive bladder cancer(MIBC) require large target margins due to bladder fill variability, inevitably resulting in increased organ-at-risk(OAR) doses and toxicity risk. CBCT-guided online adaptive radiotherapy(oART) mitigates this by improving precision with daily delineation of patient anatomy followed by personalization of target coverage and OAR sparing via dose optimized to the anatomy of the day.
Methods
Patients with MIBC(n=32) treated using CBCT-guided oART were prescribed 5100-5500cGy (adjusted for OAR constraints) to the whole empty bladder and 4000cGy to pelvic lymph nodes, over 20 fractions. Patient-specific planning target volume(PTV) margins for bladder targets were determined during CT simulation by evaluating bladder fill between scans acquired 15 minutes apart. Nodal PTV margin was 0.3cm. A total of 639 oART fractions were analyzed.
Results
Average oART session time (CBCT acquisition to treatment completion) was 31±8min. Average patient-specific bladder PTV margin was 1.0cm(range:0.5-1.5). Daily oART revealed appreciable variability in bladder fill and proximity of bowel to high-dose region (quantified as overlap volume between bowel and PTV), with average daily changes of 26±22% and 34±31%, respectively. The adaptive plan was selected over non-adapted(“scheduled”) plan in 99.8% of fractions due to improved target coverage and/or OAR sparing. The primary bladder PTV coverage goal(D95%>100%) was achieved for 19%/70% of scheduled/adapted plans. These increased to 50%/99%, respectively, for the secondary goal(D95%>95%). Plan adaptation also increased the percentage of plans satisfying OAR goals: bowel, sigmoid, and rectum D0.03cc goals achieved for scheduled/adapted plans were 89/99%, 91/96%, and 64/97%, respectively.
Conclusion
Daily oART for treatment of MIBC is a powerful tool that enhances personalization and precision by ensuring optimal dose distributions in the face of appreciable daily bladder fill and OAR variability. This facilitates use of smaller target margins and generally improves target coverage and OAR sparing compared to non-adaptive approaches.