Background/Aims: Preoperative chemoradiation therapy (CRT) is standard procedure for locally advanced rectal cancer. The correlation of tumor response evaluated using CT according to response evaluation criteria in solid tumors (RECIST) with the histological tumor regression grade (TRG) is not well-documented. Methodology: Ninety-one patients with rectal cancer underwent CT examinations before and after preoperative CRT and following surgery. Clinical tumor staging and tumor response assessed according to RECIST were done on paired CT scans. Pathological tumor staging and TRGs were reviewed in resected specimens. Post-CRT CT findings and histological findings were compared. Survival analysis for 73 patients was done. Results: TRG was positively correlated with the CT-assessed tumor response (τ=0.276, p=0.009). Thickened fibrotic areas and muscle disarray caused by fibrosis were more frequently seen in cases of patients over-diagnosed as having residual tumors. The ycT status was positively correlated with ypT status (r=0.44, p<0.001; accuracy=61.5%). Downstaging of cT status was correlated with a lower TRG (p=0.001). Conclusions: Fibrosis emerges after neoadjuvant therapy and is usually accompanied by tumor reduction on CT scans of rectal cancer patients following preoperative CRT. Therefore, tumor response assessed using CT according to RECIST may serve as a supplementary tool for preoperative planning other than tumor restaging.
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