The predictive value of metabolic response to preoperative radiochemotherapy in locally advanced rectal cancer measured by PET/CT

Background  To evaluate the value of positron emission tomography using fluorodeoxyglucose and computer tomography scan (FDG-PET/CT) for prediction of histopathological response of preoperative radiochemotherapy (RCTX) in patients with rectal carcinoma. Methods  Thirty patients with uT3 rectal carcinoma were examined by FDG-PET/CT at baseline, 14 days after initiation, and after completion of preoperative RCTX. The FDG decreases seen with PET scanning from baseline to day 14 (early metabolic response) and after completion of therapy (late metabolic response) were compared with histopathological tumor response. One patient denied surgery after RCTX. Results  The mean (±SD) reduction of tumor FDG uptake in histopathologically responding compared to non-responding tumors was −44.3% (±20.1%) versus −29.6% (±13.1%) (p = 0.085) at day 14 and −66.0% (±20.3%) versus −48.3% (±23.4%) (p = 0.040) after completion of RCTX. Best differentiation of histopathological tumor response was achieved by a cut-off value of 35% reduction of initial FDG uptake at day 14 and 57.5% after completion of therapy. Applying the cut-off values as a criterion for metabolic response, histopathological response was predicted with a sensitivity of 74% (14/19) at day 14 and 79% (15/19) after completion of therapy. The positive predictive value for early metabolic response was 82% (14/17) and for late metabolic response was 83% (15/18). Histopathological evidence of accumulated peritumoral inflammation cells was associated with a minor FDG decrease in five histopathologically responding patients, and influenced the results with negative predictive values of 58% (7/12) and 64% (7/11) at the early and late time points, respectively. Conclusions  Metabolic response to a preoperative RCTX using FDG-PET/CT in rectal cancer patients can be correlated with histopathological response, but FDG uptake of peritumoral inflammation cells limited the results and led to false negative results.