Pelvic MRI Following Total Neoadjuvant Therapy for Rectal Cancer Poorly Predicts a Complete Clinical Response

BACKGROUND AND OBJECTIVE: Determining complete clinical response (cCR) after total neoadjuvant therapy (TNT) for the treatment of rectal cancer remains challenging. Post-treatment restaging can show discordant results between endoscopy and pelvic MRI. The aim of this study was to assess the outcomes of patients that had an endoscopic cCR upon restaging, but showed continued radiographical disease. METHODS: This is a retrospective study of rectal cancer patients treated at UChicago (2015-2022). Patients who received TNT, had a cCR on restaging endoscopy, and pursued nonoperative management (NOM) were included. Outcomes between patients with residual or no residual disease on restaging MRI were compared. RESULTS: Thirty patients were endoscopically negative on restaging and entered NOM. Of these, restaging MRI showed residual disease in 12 (40%) patients and no residual disease in 18 (60%) patients. After a mean follow-up of 4.8 years, nine patients (30%) experienced regrowth. There was no difference in regrowth rates between patients with negative versus positive restaging MRI (33% vs. 27.8%; p = 0.75). After a mean of 4.0 years, 67% of patients who had an initial positive restaging MRI did not develop a regrowth. CONCLUSION: Two-thirds of endoscopically negative patients with initially positive restaging MRIs showed no regrowth after 4 years.