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 (18)F-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 (T(3-4)/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: SUV(max), SUV(avg), 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 ≥95% 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. SUV(max), SUV(avg), 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.