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

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.

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.

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.