Assessing FDG-PET/3-T MRI after Preoperative Chemoradiotherapy for Rectal Cancer

PURPOSE To assess the accuracy of pelvic 3-T MRI and combined FDG-PET/3-T MRI (PET/MRI) in predicting pathological tumor and node (ypTN) stages, and to compare the accuracy of whole-body PET/MRI with thoraco-abdominal CT (CT) in predicting metastases (ypM) stage. METHOD AND MATERIALS This prospective study concerned 17 patients (16 male) with locally advanced rectal cancer who underwent preoperative chemoradiotherapy, PET/MRI and CT for staging purposes. PET/MRI included T2 and diffusion weighted images. Total mesorectal excision was the treatment of choice for 13 patients; the remainders were MRI node negative and underwent transanal local excision with at least 1-year endoscopic and pelvic MRI follow-up. Concurrent distant metastases were confirmed by surgery/biopsy or followed up with CT. One radiologist assessed pelvic MRI and CT images. Another radiologist and a nuclear medicine physician jointly assessed PET/MRI findings. All three were blinded to all other imaging and pathology results. RESULTS ypT was T0 in 4 patients, T1 in 3, T2 in 1, T3 in 7, and T4 in 2. ypN was positive in 5/17 cases, and metastases were detected in 3/17 patients. MRI and PET/MRI findings for ypT were concordant and correct in 11/17 patients (64.7%), concordant and incorrect in 2/17 (11.8%), and discordant in 4/17 (23.5%), PET/MRI staging being correct in 2 cases. As for ypN staging, MRI and PET/MRI were concordant and correct in 14/17 patients (82.3%) and discordant in 3/17 (17.7%), with PET/MRI staging predicting ypN status in 2 cases. Two patients with metastases were diagnosed correctly, while PET/MRI misdiagnosed one case of a small lung metastasis. CONCLUSION Integrated whole-body PET/MRI improves the accuracy of ypTN staging, but is less accurate than CT in ypM staging. Further studies are needed, including efforts to refine PET/MRI by using specific sequences for the lung and intravenous gadolinium, to examine the role of this technique in monitoring distal cancer spread. If successful, it would be possible to combine local and distant rectal cancer staging in a single examination. CLINICAL RELEVANCE/APPLICATION FDG-PET/3-T MRI can be a useful tool for the whole-body staging (TNM) of patients with advanced rectal cancer after chemoradiotherapy.