Microscopic spread of low rectal cancer in regions of the mesorectum: detailed pathological assessment with whole-mount sections

Background and aims: The aim was to investigate the regional spread of microscopic tumor nodules in the mesorectum of patients with low rectal cancer, and to provide further pathological evidence for optimal procedure selection of radical resection for rectal cancer. Patients and methods: Sixty-two patients with low rectal cancer underwent low anterior resection and total mesorectal excision (TME). Surgical specimens were sliced transversely on serial embedded blocks at 2.5-mm intervals, and stained with hematoxylin and eosin. On whole-mount sections the mesorectum was divided into three regions: the outer region of the mesorectum (ORM), the middle region of the mesorectum (MRM), and the inner region of the mesorectum (IRM). Microscopic metastatic foci were investigated for metastatic mesorectal region, frequency, types, involvement of the lymphatic system, and correlation with the primary tumor. Results: Microscopic spread of the tumor was observed in 38.7% of patients ( 24 out of 62) and that in the ORM was observed in 25.8% of the patients ( 16 out of 62). Microscopic tumor foci spread in the circumferential resection margin (CRM) occurred in 6.5% of the patients ( 4 out of 62), and distal mesorectum (DMR) involvement was detected in 6.5% ( 4 out of 62), with the spread extending to within 3 cm from the lower margin of the tumor. Conclusions: The results of the present study support the theory that complete excision of the mesorectum without destruction of the ORM is essential for surgical management of low rectal cancer, and an optimal DMR clearance resection margin should be no less than 4 cm. Four patients with microscopic tumor nodule spread in the ORM observed in the study suggested that microscopic metastases exist in pelvic lateral areas and in the mesorectum simultaneously, indicating the significance of preoperative and/or postoperative radiochemotherapy.