MRI and FDG-PET for Assessment of Response to Neoadjuvant Chemotherapy in Locally Advanced Rectal Cancer

The purpose of this study was to assess the value of magnetic resonance imaging (MRI) and additional F-18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) for tumor response to neoadjuvant chemotherapy (NAC) in patients with locally advanced rectal cancer (LARC). Data on 40 patients with LARC, who were treated with NAC and underwent MRI and FDG-PET/CT before and after NAC, were analyzed retrospectively. Surgery was performed at a median of 6 weeks after NAC and the images were compared with the histological findings. The tumor regression grade 3/4 was classified as a responder. Sixteen patients were pathological responders. Receiver operating characteristic (ROC) analysis revealed that MRI total volume after NAC (MRI-TV2) and Delta MRI-TV had the highest performance to assess responders (area under the ROC curve [AUC] 0.849 and AUC 0.853, respectively). The reduction rate of the maximum standardized uptake value (Delta SUVmax) was also an informative factor (AUC 0.719). There seems no added value of adding FDG-PET/CT to MRI-TV2 and Delta MRI-TV in assessment of NAC responders judging from changes in AUC (AUC of Delta SUVmax and MRI-TV2 was 0.844, and AUC of Delta SUVmax and Delta MRI-TV was 0.846). MRI-TV2 and Delta MRI-TV were the most accurate factors to assess pathological response to NAC. Although Delta SUVmax by itself was also informative, the addition of FDG-PET/CT to MRI did not improve performance. Patients with LARC who were treated by induction chemotherapy should receive an MRI examination before and after NAC to assess treatment response. A more than 70 % volume reduction shown by MRI volumetry may justify the omission of subsequent radiotherapy.