Purpose: Early detection of a response to neoadjuvant chemotherapy for locally advanced rectal cancer may spare patients from additional toxic but ineffective chemotherapy. Using 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET), we evaluated tumor response prospectively in the early course of preoperative chemotherapy. Methods: The subjects were 15 patients who received neoadjuvant chemotherapy (XELOX or XELOX plus bevacizumab) for locally advanced rectal cancer. Patients underwent 18F-FDG PET before chemotherapy, at the end of the first cycle of chemotherapy, and before surgical resection. Magnetic resonance imaging (MRI) was performed before chemotherapy, after the second cycle of chemotherapy, and before resection. After resection, the SUVmax and diameter were compared and graded according to the tumor regression grade (TRG). Results: The TRG was assessed as TRG1 in one patient, TRG2 in five patients, and TRG3 in nine patients. We divided the patients into two groups: non-responders (NR) included the TRG1 and TRG2 patients, and responders (R) included the TRG3 patients. The tumor size before surgery was significantly smaller in the R group than in the NR group. The SUVmax at the end of the first cycle of chemotherapy and before surgical resection was significantly lower in the R group than in the NR group. Conclusion: Performing 18F-FDG PET at the end of the first cycle of chemotherapy allowed us to predict the pathological response of locally advanced rectal cancer.
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