3015 - CBCT-Guided Online Adaptive Radiotherapy for Rectal Cancer: Initial Dosimetric and Clinical Experience

A. W. Lee¹, Y. F. Wang¹, D. P. Horowitz², and L. A. Kachnic³; ¹Department of Radiation Oncology, Columbia University Irving Medical Center, New York, NY, ²Columbia University Irving Medical Center, New York, NY, ³Columbia University, New York, NY

Purpose/Objective(s): Daily cone-beam (CB) CT-guided online adaptive radiotherapy (ON-ART) utilizes an effective AI platform and robust imaging to tailor plans to account for daily variation in tumor size and patient anatomy. Adapted plans that improve planning target volume (PTV) coverage and minimize dose to organs at risk (OAR) may be beneficial in total neoadjuvant therapy (TNT) for rectal cancer in the era of watchful waiting. Here, we report the first US clinical experience of CBCT-guided ON-ART for rectal cancer. Materials/

Methods: This is a single institution retrospective analysis of patients with stage II/III rectal cancer with CBCT-guided ON-ART prior to systemic therapy. Patients were planned with a non-contrast CT in the prone position on bowel displacement board with a full bladder (150-200 cc). The primary tumor and suspicious nodes received 50 Gy in 25 fractions; elective nodes received 45 Gy in 25 fractions. Residual disease received a 4 Gy boost in 2 fractions. Oral capecitabine, 825 mg/m2 twice daily, was administered on days of RT. Plans were created utilizing our rectal ON-ART template. For each fraction, clinical target volume (CTV), PTV and OAR volumes were recontoured with AI assistance after CBCT. Standard CTV/PTV margins were used. The adapted (A) plan was calculated, underwent QA, and then compared to the scheduled (S) plan. Using pre-specified dosimetric triggers, physicians would select the superior plan. This plan would be delivered following repeat CBCT. CTCAEv5.0 toxicities were recorded. Statistical analysis of PTV and OAR dosimetry was performed using a two-sided t-test and an a level of 0.05.
Results: Between April 2023 and December 2023, 10 patients (270 fractions) were analyzed. The mean time from initial CBCT to RT completion was 22 minutes (range: 18-27). The mean physician time required to recontour, assess, and select daily plans was 17 minutes (range: 13-20).

Only 20 (7.4%) scheduled plans were selected for treatment over the adapted plan. Dosimetric data for PTV coverage and OAR dose can be found in Table 1.

Treatment was well tolerated with only one grade 3+ toxicity (thrombocytopenia in a patient with underlying idiopathic thrombocytopenic purpura). Thus far, 3 patients have achieved complete clinical response, while the others continue TNT chemotherapy.
Conclusion: Daily CBCT-guided ON-ART significantly increased PTV coverage and decreased dose to the bowel. Treatment was well tolerated with only 1 patient experiencing grade 3+ toxicity. Our initial clinical experience demonstrates ON-ART to be safe and feasible in the treatment of rectal cancer.

Abstract 3015 – Table 1

	Scheduled	Adapted	p-value
PTV_45 V98% > 98%	96.34	99.27	<0.001
PTV_50 V98% > 98%	92.56	99.39	<0.001
PTV_54 V98% > 98%	96.34	99.27	0.013
Bowel D0.03cc < 5200 cGy	5000.30	4922.40	<0.001
Bowel V45 < 100 cc	88.42	69.10	0.357
Bladder D0.03cc < 5200 cGy	4685.60	4823.00	<0.001
Bladder D50% < 3500 cGy	3201.20	3118.00	0.446