Simulated Cone-Beam Computed Tomography Based Online Adaptive Radiotherapy of Anal Cancer - Potential Dosimetric Benefit

PURPOSE/OBJECTIVE(S): To evaluate the dosimetric benefit of cone beam computed tomography (CBCT) based online adaptive radiotherapy (oART) with full re-optimization on the anatomy of the day, for patients with anal cancer. MATERIALS/METHODS: CBCT-based oART was simulated using a novel commercial solution in research mode. Twenty-three consecutive patients with anal cancer, treated with radiotherapy from January to November 2020, were retrospectively included. Treatments were delivered in 30 fractions with a prescribed dose of 60 and 48 Gy to the primary and elective targets, respectively. Sixteen patients were also prescribed 60 Gy to positive lymph nodes. Two reference treatment plans were generated based on the planning CT for each patient; one with clinically used clinical target volume (CTV) to planning target volume (PTV) margins for image guided radiotherapy (non-ART) and one with expected clinical oART CTV-PTV margins. The oART margins were reduced 5 mm isotropically and 5 mm anteriorly for the positive lymph nodes and elective target, respectively, compared to non-ART; corresponding to a 50 % reduction from non-ART. Treatment sessions were simulated on six weekly CBCT images for one of the patients. Bladder, rectum and bowel cavity structures were automatically delineated and manually adjusted to match the anatomy on the CBCT. For non-ART, the reference plan was re-calculated on the anatomy of the day with rigidly propagated targets. For oART, the plan was re-optimized using targets that were either deformed or rigidly propagated, according to the expected clinical workflow. The volume of bowel cavity receiving 30 and 45 Gy (V30Gy and V45Gy, respectively) was evaluated and compared between the two workflows. RESULTS: For the reference treatment plans, the CTV-PTV margin reduction in oART resulted in a median reduction (interquartile range) for bowel cavity V30Gy and V45Gy of 6.4 (3.8 - 8.5) % and 11.4 (9.7 - 14.0) %, respectively. This corresponded to 39.4 (17.9 - 80.2) cc and 48.5 (35.6 - 61.3) cc, respectively. Re-optimizing the plan to the anatomy of the six CBCT images lead to a median reduction [minimum; maximum] for bowel cavity V30Gy and V45Gy of 9.2 [7.5; 12.3] % and 13.1 [10.8; 15.3] %, respectively, corresponding to 58.0 [46.5; 86.0] cc and 56.9 [46.4; 70.3] cc. CONCLUSION: Simulated oART of anal cancer resulted in reduced dose to the bowel cavity, indicating potential reduction in gastrointestinal toxicity. Early results demonstrated that the superiority of oART compared to non-ART was consistent during the course of treatment. Further oART sessions of other patients are planned to be simulated before the ASTRO conference. A single-arm phase II trial to investigate the clinical impact has been developed and awaits approval.