Evaluating the usefulness of 18F-FDG PET/MR in rectal cancer staging: a prospective comparison with histopathological findings

BACKGROUND: Accurate preoperative staging of rectal cancer (RC) is essential for treatment planning. MRI is the standard for local assessment but has limitations in evaluating nodal involvement and tumor deposits. Hybrid PET/MR combines morphological and metabolic imaging and may improve staging accuracy. This study assessed PET/MR performance in RC and regional lymph nodes staging compared with histopathology. METHODS: Sixteen patients with biopsy-proven rectal cancer underwent PET/MR with ^18F-FDG. Tumor size, T stage, nodal status, and tumor deposits were evaluated using multiparametric MR sequences (T1, T2, diffusion-weighted, contrast-enhanced) combined with PET. Imaging findings were compared to histopathological results. Diagnostic performance metrics, including accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), were calculated with corresponding 95% confidence intervals. RESULTS: PET/MR accurately staged the primary tumor in 81.25% of cases (95% CI: 57.0-93.4), with sensitivity of 100%, specificity of 50.0%, PPV of 76.9%, and NPV of 100% for advanced T-stage assessment. Three cT3 tumors were downstaged to pT2; no tumors were upstaged. For nodal staging based on ESGAR criteria, diagnostic accuracy was 75.0% (95% CI: 50.5-89.8), with sensitivity of 57.1%, specificity of 88.9%, PPV of 80.0%, and NPV of 72.7%. Tumor deposits were not detected on imaging but were present in four specimens. Incidental findings included a biopsy-confirmed breast adenocarcinoma and one benign thyroid lesion. CONCLUSION: 18F-FDG PET/MR shows potential for accurate assessment of the primary tumor and reliable exclusion of advanced T-stage disease. However, its performance in nodal staging and tumor deposit detection remains limited. Given the small sample size and potential confounding from preoperative radiotherapy, these findings should be considered hypothesis-generating and require validation in larger prospective cohorts.