Investigation into the effect of neoadjuvant therapy and tumor regression grade on the shrinkage of distal surgical margin in rectal cancer: A prospective case-control study

Background: The present study aimed to explore the effect of neoadjuvant therapy and tumor regression grade (TRG) on the shrinkage in the distal surgical margin ( DSM) induced by formalin fixation in rectal cancer. Materials and Methods: In this prospective study, the DSM of resected 61 specimens of rectal and rectosigmoid junction adenocarcinoma were measured following fresh and formalin fixation. The measurements were performed within the first 15 min after resection and at 24 h after formalin fixation without pinning and were compared with regard to neoadjuvant treatment status and TRG. Results: In the patients that received neoadjuvant therapy, the fresh and postfixation DSM values were 32.2 mm and 22.7 mm, respectively, and the mean shrinkage rate was 34.7% (P. 0.001). In the patients that did not receive neoadjuvant therapy, the fresh and postfixation DSM values were 54.03 mm and 41.9 mm, respectively, and the mean shrinkage rate was 23.7% (P. 0.001). The mean shrinkage rate was 41.9% in TRG 1, 29.4% in TRG 2, and 31.9 in TRG 3 specimens. The mean shrinkage rate was higher in specimens with a DSM of =20 mm compared to specimens with a DSM of >20 mm (46.2% vs. 24.9%). Conclusion: A complete or near-complete tumor regression in patients with rectal cancer undergoing neoadjuvant therapy increases the shrinkage of DSM. Moreover, this shrinkage rate is likely to be higher and the pathological DSM is likely to be closer than expected in cases that present a better clinical response to neoadjuvant therapy, particularly in distal rectal cancer.