Response Prediction of Chemoradiotherapy for Rectal Cancer Using Rapid Semi-Automated Flow Cytometry

(1) Background: The effectiveness of chemoradiotherapy (CRT) for rectal cancer varies greatly among patients, and no reliable biomarker for predicting treatment response currently exists. We investigated whether cell cycle analysis using rapid semi-automated flow cytometry could predict the effectiveness of CRT. (2) Methods: This prospective study included 32 patients with rectal cancer undergoing CRT. The cell cycle of biopsy specimens from rectal cancer patients before CRT was semi-automatically and rapidly analyzed using Celltac PEAK as flow cytometer. The proportion of cell cycle phases is compared with treatment response after CRT. (3) Results: Patients with a tumor reduction rate ≥ 30% on CT showed a significantly higher proportion of cells in the Over G2/M phase compared with those with <30% reduction (median 2.45% vs. 0.95%, <i>p</i> = 0.022). Similarly, tumors demonstrating disappearance of FDG uptake after CRT had a significantly higher proportion of cells in the Over G2/M phase compared with those with persistent FDG uptake (median 4.05% vs. 1.24%, <i>p</i> = 0.024). (4) Conclusions: A high proportion of cells in the Over G2/M phase could be a potential biomarker for predicting tumor shrinkage in CRT for rectal cancer.