Change in glucose metabolism measured by 18F-FDG PET/CT as a predictor of histopathologic response to neoadjuvant treatment in rectal cancer

In order to analyze the changes of glucose metabolism by maximum standardized uptake value (SUVmax) of 18F-FDG PET/CT in patients with rectal cancer submitted to neoadjuvant radiochemotherapy (nRCT) and to correlate SUV changes with tumor regression grade (TRG). Three sequential 18F-FDG PET/CT studies were performed in 31 patients with rectal cancer at the following time point: before starting the treatment (PET/CT1), during the treatment (PET/CT2), and after completion of neoadjuvant treatment (PET/CT3). The SUVmax values of the rectal lesion in the PET/CT1 (SUV1), PET/CT2 (SUV2), and PET/CT3 (SUV3) were obtained; deltaSUV1 [(SUV1 - SUV2)/SUV1] and deltaSUV2 [(SUV1 - SUV3)/SUV1] were also calculated. Metabolic parameters were compared to TRG. Significant differences in pathologic responder and non-responder patients were found only for SUV2 (6.4 +/- A 2.9 in responder and 10.7 +/- A 4.8 in non-responder patients, respectively; P = 0.006) and SUV3 (3.6 +/- A 1.4 in responder and 6.6 +/- A 2.1 in non-responder patients, respectively; P = 0.0009). The best predictor for TRG response was SUV3 (threshold of 4.4) with sensitivity, specificity, accuracy, negative predictive value, and positive predictive value of 77.3%, 88.9%, 80.7%, 61.5%, and 94.4%, respectively. 18F-FDG PET/CT is a reliable and accurate technique to assess the response to nRCT in rectal cancer. In our population, the absolute value of SUVmax after treatment was the best predictor of pathological response.