Early detection to neoadjuvant chemotherapy response in locally advanced rectal cancer using 18F-FDG PET.

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440 Background: Early detection of treatment response of neoadjuvant chemotherapy (CTx) for locally advanced rectal cancer may spare patients the toxicity of ineffective CTx. We evaluated prospectively tumor response with 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) in the early course of preoperative CTx. Methods: A total of 15 patients with locally advanced rectal cancer (T3 or T4 and N0-N2) received neoadjuvant CTx (XELOX or XELOX+bevacizumab). Patients underwent 18F-FDG PET before CTx, at the end of first cycle of CTx (12-38 days: median 15days), and before surgical resection. Scans were interpreted using SUVmax parameter. Magnetic resonance imaging (MRI) was undergone before CTx, at the end of 2 cycles of CTx (25-54 days: median 38 days), and before resection. The tumor longest diameter was measured by MRI T2-weighted images. After resection, SUVmax and diameter were compared with the Tumor Regression Grade (TRG). Results: Of 15 patients, TRG1 was in 1 patient, TRG2 was in 5 patients, and TRG3 was in 9 patients. We divided into 2 groups, non-responder (NR) group was TRG1 and TRG2, and responder (R) group was TRG3. There was no significant difference as for clinicopathological parameters before CTx including tumor diameter and SUVmax. The tumor size before surgery was significantly smaller in R group than NR group (22.2 mm vs. 35.0 mm; p = 0.017). SUVmax in R group was significantly lower at the end of first cycle of CTx (4.9 vs. 9.3; p = 0.023), and before surgical resection (3.2 vs. 10.3; p = 0.007) than that in NR group. Conclusions: In this study, 18F-FDG PET at the end of first neoadjuvant CTx successfully predict pathological response of locally advanced rectal cancer.