3518 - Reduced Time to Start with Radiotherapy Treatment Using Online Adaptive Radiotherapy without Planning CT Scan: A Prospective Clinical Trial in Patients with Rectal Cancer

Purpose/Objective(s): Preoperative radiotherapy is effective for patients with stage II and III rectal cancer. In our department, the average time from initial consultation to the start of treatment was 15 days, primarily due to the scheduling of a planning CT scan and preparation of a radiation treatment plan. This study aimed to reduce the time to treatment to =7 days by using an online adaptive radiotherapy workflow based on the diagnostic CT scan instead of a planning CT scan. Secondary objectives included evaluating the workflow’s timing and plan characteristics, as well as patient-reported quality of life.

Materials/Methods: Patients requiring preoperative (chemo)radiotherapy for rectal cancer were eligible. The workflow started with consultation by a radiation oncologist. The diagnostic CT scan was used for preplanning, on which a reference treatment plan was made. Throughout the radiotherapy treatment, before, during and directly after each treatment fraction, a high-quality cone beam CT scan was made, which enabled online contour and treatment plan adaptation. The evaluation of the adaptive workflow consisted of description of on-coach treatment time (time in minutes between first and third CBCT). Plan characteristics were evaluated by analyzing dosimetric data of the adapted and initial reference treatment plan. Patient-reported quality of life was assessed with questionnaires at baseline, at the end of treatment and 8-12 weeks after end of treatment.

Results: Twenty patients were enrolled, with a mean treatment time of 5.45 days. A total of 320 treatment fractions were delivered, with a median on-couch treatment time of 26 minutes. The AI-generated clinical target volume (CTV) required adaptation in 190 fractions (59.38%), leading to a longer median treatment time of 28 minutes compared to 22 minutes for fractions without CTV adjustment (p < 0.001). The adapted plans showed adequate PTV and CTV coverage, with significant dosimetric improvements over initial reference plans. Notably, (214 fractions) 66.9% of initial reference treatment plans did not meet CTV coverage constraints when applied to daily anatomy.

Conclusion: Treatment of patients with stage II and III rectal cancer based on an initial plan on a diagnostic CT scan and a daily online adaptive workflow was feasible. It resulted in a reduced time to treatment. The adapted treatment plan better matched the changing daily anatomy of the patient, resulting in a significantly better coverage of the target volumes and an improved sparing of organs at risk.