18F-FDG PET/MRI for Rectal Cancer TNM Restaging After Preoperative Chemoradiotherapy: Initial Experience

OBJECTIVE: F-18-FDG-PET/MRI is a novel hybrid techinque that has been recently introduced in oncological imaging, showing promising results. The aim of this study is to assess the value of whole-body F-18-FDG-PET/MRI for predicting the pathological stage of locally advanced rectal cancer after preoperative chemoradiotherapy. DESIGN: This was a prospective observational study. SETTINGS: The study was conducted at a tertiary care hospital. PATIENTS: Thirty-six patients with locally advanced rectal cancer (25 male, median age 68.5 years) were prospectively assessed with PET/MRI and thoracoabdominal CT before and after preoperative chemoradiotherapy. Twenty-seven patients underwent low anterior or abdominoperineal resection. Nine patients with a complete clinical response underwent organ-preserving treatment (8 local excision and 1 watch-and-wait approach) with > 1-year follow-up. MAIN OUTCOME MEASURES: One radiologist evaluated pelvic MRI and CT. A second radiologist and a nuclear medicine physician jointly assessed PET/MRI. The imaging was compared with histology or follow-up (ypT0 vs T >= 1 and ypN0 vs ypN+ categories). Metastases were confirmed with biopsy or a follow-up CT scan at least at 1 year after preoperative chemoradiotherapy. The sensitivity, specificity, and accuracy values of the imaging techniques were calculated using standard formulas. RESULTS: The accuracy for ypT staging was 89% and 92%, and the accuracy for ypN was 86% and 92% for MRI and PET/MRI. Compared with CT, PET/MRI correctly diagnosed 4 of 5 metastases, but it did not detect a lung metastatic nodule. In 11% of the patients, the PET/MRI changed the treatment strategy. LIMITATIONS: This study is limited by its small sample size. CONCLUSIONS: Although the whole-body PET/MRI was more accurate than the pelvic MRI alone for the prediction of tumor and node response to preoperative chemoradiotherapy, the technique performed worse than CT in detecting small lung metastasis.