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To assess the diagnostic accuracy of 18F-FDG-PET/CT and MRI alone and together inpredicting tumour invasion within the peritoneal reflection, central, inferior, lateral, posterior and anterior compartments above and below the peritoneal reflection.Twenty-four consecutive patients (15 males; median age 58) withpre-operative images of both 18F-FDG-PET/CT and MRI, who underwent exenterative surgery for recurrent and locally advancedprimary rectal cancer, were included. Two radiologists and two nuclear medicine physicians reported the scans, blinded to clinical and other radiological findings. Diagnostic accuracy tests were calculated using histopathology as the Gold Standard.PET/CT was highly accurate inpredicting tumour invasion in all compartments (AUC > 77.5) except the posterior [Area under curve (AUC) = 69.3%] and anterior compartment below the peritoneal reflection (AUC = 69.3%). MRI was also less accurate for these compartments (AUC = 79.8% and 78.8% respectively). This was due to PET/CT's low sensitivity (63.6% and 52.9% respectively) and MRI's low specificity (66.7% and 71.4% respectively). Using both modalities, the AUC for these compartments increased to 86.4% and 97.1% respectively. Combining both modalities increased the AUC for all compartments up to 18%.The combined use of MRI and PET/CT substantially increases diagnostic accuracy inpredicting recurrent or locally advancedprimary rectal tumour invasion within the pelvis.