Background. Magnetic resonance imaging (MRI) is highly accurate in local staging of rectal cancer. It can identify features known to be associated with increased risk of metastatic disease. We evaluated the incidence of synchronous metastatic disease on fludeoxyglucose-positron emission tomography/computed tomography (FDG-PET/ CT) and contrast-enhanced multiple-row detector computed tomography (ceMDCT) in MRI-stratified high- and low-risk rectal cancers. The aim was to determine the incidence of synchronous metastatic disease according to MRI risk features. Methods. A total of 236 patients with rectal cancer were recruited to a study evaluating FDG-PET/CT. All patients underwent MRI staging and were stratified into high and low risk (high risk: extramural venous invasion, extramural spread of >5 mm or T4, involved circumferential resection margin or intersphincteric plane involved for low rectal tumors). Confirmed metastases were those identified on FDG-PET/CT and ceMDCT. Results. Imaging data were available for 230 (97.5%) of 236 patients. Incidence of confirmed distant metastases was significantly greater in the MRI high-risk group, with 28 (20.7%) of 135 (95% confidence interval [CI] 14.8-28.3), versus the low-risk group, with 4 (4.2%) of 95 (95% CI 1.7-10.3) (odds ratio 6.0, 95% CI 2.0-17.6, P<0.001). Conclusions. Adverse features found on rectal MRI identify patients at increased risk of synchronous metastatic disease. This group may benefit from additional preoperative investigation for synchronous metastases such as FDG-PET/CT or liver MRI and from alternative neoadjuvant chemotherapy regimens including induction chemotherapy.
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