The Predictive Role of Sequential FDG-PET/CT in Response of Locally Advanced Rectal Cancer to Neoadjuvant Chemoradiation

Abstract

This study prospectively assessed the value of sequential fluorine-18-fluorodeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) scans for predicting the response of locally advanced rectal cancer to neoadjuvant chemoradiation. Fifty consecutive patients with locally advanced rectal cancer were enrolled. The treatment consisted of concurrent chemoradiation, which included preoperative 5-fluorouracil-based chemotherapy and pelvic radiation (4500 to 5040 cGy); this was followed 8 weeks later (median: 55 d) by surgery with a curative intent. All the patients underwent FDG-PET/CT before and 5 weeks later (median: 35 d) after the completion of chemoradiation. We evaluated the measurements of the FDG uptake [maximum standardized uptake values (SUV(max))] and the percentage of SUV(max) difference [(response index (RI)] between the prechemoradiation and postchemoradiation FDG-PET/CT scans. After chemoradiation, 32 of 50 patients (64%) were classified as responders according to the tumor regression grade, which is based on the ratio of fibrosis to residual cancer (tumor regression grade 3-4). For all the patients, the mean prechemoradiation SUV(max) was 14.8, and this was significantly higher than the mean SUV(max) value of 6.1 at postchemoradiation (P< 0.001). The mean RI was significantly higher in the responders than that in the nonresponder patients (62.6% vs. 31.2%, P = 0.001). The following parameters were obtained using a RI cutoff of 53.0% for defining a response to therapy: 75.0% sensitivity, 73.2% specificity, 82.8% positive predictive value, and 61.9% negative predictive value. The FDG-PET/CT parameters, and especially the RI, may be best for assessing the neoadjuvant chemoradiation response of locally advanced rectal cancer and these values can potentially assist physicians for planning the optimal treatment.