2.2  Endoscopic assessment of tumor response before and after preoperative CRT
All endoscopic procedures were performed by gastrointestinal endoscopists with more than 5 years’ experience. A pre-CRT endoscopy, accompanied by tissue biopsy, was performed before initiating preoperative CRT. Four weeks after the completion of preoperative CRT, endoscopic tumor response was evaluated prior to surgery (Fig. 1). All reviewers were blinded for histology and each other's results. Under same categorical criteria, all reviewers conducted reassessment of tumor response considering the tumor size, morphology, and involved intraluminal circumference before surgery. Endoscopic tumor response was categorized as follows: cCR, no visualization of tumor, white or red scar; nearly-cCR, minimal residual nodularity or stenosis; non-cCR, any ulcer with a necrotic bed regardless size, a definite residual mass, or nodularity (Fig. 2A).
Figure 2  (A) Tumor on endoscopy before and after preoperative chemoradiation therapy (pre-CRT). (a–d) Primary tumor before pre-CRT. (e, f) Clinically complete response of primary tumor after completion of pre-CRT; a typical sign of clinical complete response (black arrows indicate a red scar and a white scar). (g) Minimal residual nodularity and (h) residual tumor with large necrotic area after pre-CRT (a typical sign of poor tumor response). (B) Magnetic resonance imaging (MRI) scans from patients before and after pre-CRT. (a–d) Primary tumor before pre-CRT. (e) Tumor regression grade (TRG) 1; the green arrow indicates radiologic complete response with a hypointense rectal wall. (f) TRG 2; the green arrow indicates a slightly thickened hypointense rectal wall. (g) TRG 3; the green arrow indicates a visible intermediate signal. (h) TRG 4; the green arrow indicates a small area of fibrosis.