Comparison of restaging accuracy of repeat FDG-PET/CT with pelvic MRI after preoperative chemoradiation in patients with rectal cancer

PURPOSE: The aim of this study was to compare the restaging accuracy of repeat fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) scan with pelvic magnetic resonance imaging (MRI) in patients with rectal cancer who have undergone preoperative chemoradiation.

METHODS: One hundred and eighty-one consecutive patients with locally advanced rectal cancer who underwent a total mesorectal excision after preoperative chemoradiation were prospectively enrolled. All the patients underwent FDG-PET/CT and pelvic MRI before chemoradiation and 5 weeks after the completion of chemoradiation. We evaluated the measurements of the FDG uptake (SUV(max)) and the percentage of SUV(max) difference (Response Index = RI) between the pre- and postchemoradiation FDG-PET/CT scans. The accuracy of repeat FDG-PET/CT and pelvic MRI for predicting pathologic CR were compared.

RESULTS: The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of pelvic MRI for predicting pathologic CR were 38.5, 58.1, 13.3, 84.9, and 55.2%, respectively. In terms of FDG-PET/CT, pretreatment tumor size and pathologic stage were significantly correlated with the RI values. Using a RI value of 63.6% as the cutoff threshold, it was possible to discriminate the CR from the non-CR with a sensitivity of 73.1%, a specificity of 64.5%, a PPV of 25.7%, a NPV of 93.5%, and an accuracy of 65.7% (area under the curve = 0.723, 95% confidence interval 0.619-0.828, P < 0.001).

CONCLUSIONS: The accuracy of FDG-PET/CT restaging is superior to that of MRI staging for predicting pathologic CR in irradiated rectal cancer. An NPV of 93.5% indicates that FDG-PET/CT can rule out the pathologic CR.