Optimal Interval for 18F-FDG-PET After Chemoradiotherapy for Rectal Cancer

Patients with rectal cancer often undergo chemoradiotherapy (CRT) before their tumors are surgically removed. The efficacy of CRT is determined using imaging via F-18-fluorodeoxyglucose positron emission tomography (FDG-PET) before surgery. We found that waiting at least 7 weeks after CRT completion before subjecting patients to FDG-PET is best for predicting pathological good response before surgery. Our data ought to enhance the guidelines related to evaluating CRT efficacy in patients with rectal cancer before undergoing surgical excision. Introduction: Although F-18-fluorodeoxyglucose (FDG)-positron emission tomography (PET) has been increasingly used to evaluate the response to preoperative chemoradiotherapy (CRT) in patients with rectal cancer, the optimal intervals between completion of CRT, PET, and surgery have not been fully investigated. Patients and Methods: A total of 148 consecutive patients with rectal adenocarcinoma who received CRT followed by FDG-PET and radical surgery were retrospectively analyzed. The association between the FDG-PET maximum standardized uptake value (SUVmax) and pathological response was assessed using a logistic regression model, with a primary focus on the intervals between CRT and PET as well as between PET and surgery. Results: The baseline SUVmax showed no association with pathological response (P = .201; area under the curve [AUC] = 0.528), whereas the SUVmax after CRT completion showed a strong association (P < .001; AUC = 0.707). Logistic regression analysis revealed that the ability of the SUVmax to accurately predict pathological good responders was significantly associated with a long CRTePET interval (>= 7 weeks; P = .027), but was not affected by the length of PET-surgery interval. In patients with a short CRTePET interval (< 7 weeks), the ability of the SUVmax to predict good responders was poor (P = .201; AUC = 0.669); however, in patients with long intervals (>= 7 weeks), the predictive ability markedly improved (P < .001; AUC = 0.879). Conclusion: A minimum wait time of 7 weeks is recommended before performing FDG-PET after neoadjuvant CRT for rectal cancer to obtain maximal predictive accuracy for pathological response.