MRI is more accurate than FDG-PET in assessing complete response in rectal cancer patients after neoadjuvant therapy.

Purpose: The role of FDG-PET in the restaging rectal cancer following neoadjuvant therapy (NAT) is not clear. We compared the accuracy of FDG-PET and MRI in the assessment of rectal cancer response to NAT.

Methods: Data of patients treated between January 2015 and September 2022 were captured from a rectal tumor registry. Restaging FDG-PET and MRI were evaluated for the presence of viable tumor. Imaging was compared to the reference standard of pathological results for patients that underwent surgery, and sustained clinical complete response for patients that entered watch and wait. Sensitivity was defined as correctly identifying patients with a complete response.

Results: Eighty-two patients met the inclusion criteria. Of these, 60 patients underwent restaging MRI and 54 underwent restaging FDG-PET. Thirty-two were evaluated by both modalities. Mean age and distance from anal verge were 59.9 ± 12.7 years and 5.9 ± 3.2 cm. Baseline staging was cT1-2, cT3 and cT4 for 7 (8.5%), 62 (75.6%) and 13 (15.9%) of the patients, respectively. Baseline nodal staging was cN0 and cN + for 32 (39%) and 50 (61%) of the patients, respectively. All patients were treated with radiation with the majority 73 (89%) receiving chemoradiotherapy. There were 17 patients (21%) that had a pathological or sustained clinical complete response. All baseline characteristics were not meaningfully different between groups. MRI was more accurate than FDG-PET in all parameters including sensitivity, specificity, positive and negative predictive value and overall accuracy.

Conclusion: MRI outperforms FDG-PET in the identification of complete response in rectal cancer patients after NAT.