Evaluation of 18F-FDG-PET for Early Detection of Suboptimal Response of Rectal Cancer to Preoperative Chemoradiotherapy: A Prospective Analysis

Background. Early identification of inadequate response to preoperative chemoradiotherapy (CRT) may spare rectal cancer patients the toxicity of ineffective treatment. We prospectively evaluated tumor response with F-18-fluorodeoxyglucose (FDG) positron emission tomography (PET) early in the course of preoperative CRT. Methods. A total of 27 prospectively accrued patients with locally advanced rectal cancer (T3-4/N-1) received preoperative CRT (5040 cGy + 5FU-based chemotherapy). Patients underwent PET scanning before and 8-14 days after commencement of CRT. Scans were interpreted using 3 standard parameters: SUVmax, SUVavg, and total lesion glycolysis (TLG) as well as an investigational parameter: visual response score (VRS). Percent pathologic response was quantified as a continuous variable. All PET parameters were correlated with pathology. Pathologic complete/near-complete response was defined as a parts per thousand yen95% tumor destruction, suboptimal response as < 95%. Statistical analysis was performed using the Wilcoxon rank sum test and receiver operating characteristic (ROC) curve analysis. Results. Of the 27 patients, 11 (41%) had pathologic complete/near-complete response; 16 (59%) had suboptimal response. SUVmax, SUVavg, and TLG did not discriminate between responders and nonresponders. Visual response score (VRS) was statistically significantly higher for complete/near-complete responders than for suboptimal responders (65 vs. 33%, P = 0.02). Suboptimal responders were identified with 94% sensitivity and 78% accuracy using a VRS cut-off of 50%. Conclusions. In this pilot study, FDG-PET at 8-14 days after the beginning of preoperative CRT was unsuccessful at predicting pathological response with enough accuracy to justify an early change in therapy.