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

Although ¹⁸F-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.

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.

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 CRT-PET interval (≥ 7 weeks; P = .027), but was not affected by the length of PET-surgery interval. In patients with a short CRT-PET 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).

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.