Why did we encounter a pCRM-positive specimen whose preoperative MRI indicates negative mesorectal fascia involvement in middle to low rectal cancer?

Background: This study aims to examine why we encounter a pathological circumferential resection margin (pCRM)-positive specimen whose preoperative MRI indicates negative mesorectal fascia involvement in middle to low rectal cancer.

Methods: Forty-four consecutive patients included in this study had c(yc)T1-3 primary rectal adenocarcinoma without mesorectal fascia involvement and underwent laparoscopic total mesorectal excision (TME) with curative intent in the Department of Gastrointestinal Surgery of Kansai Medical University Hospital from January 2014 to April 2018. We adopted three checkpoints to investigate the misleading point causing positive pCRM (≤ 1 mm). (1) c(yc)CRM diagnosis by two radiologists with more than 20 and 15 years of experience in rectal cancer MRI diagnosis. (2) The specimen was assessed using the TME score presented by Nagtegaal. (3) We compared the standard sectioning according to UK guidelines (group A; n = 26) with the specimen MRI image navigation-based section (group B; n = 18) in terms of estimation of pCRM by c(yc)CRM.

Results: We achieved a "complete" resection specimen in all cases. A simple correlation coefficient in group B revealed a significant correlation between c(yc)CRM and pCRM (r = 0.663, p = 0.00513); this correlation was not significant in group A (r = 0.261, p = 0.19824). However, tests for differences between linear regression coefficients in groups A and B showed no significant differences (p = 0.12596). There were five cases of pCRM ≤ 1 mm: three in group A and two in group B. An anterior lesion caused pCRM ≤ 1 mm in three cases; the tumor deposits or extramural vascular invasion caused the other cases.

Conclusion: The cause of misleading pCRM was the inaccurate preoperative MRI diagnosis of c(yc)CRM.