Aim: To define the distribution, size, location and metastasis of lymph nodes (LNs) within the mesorectum from rectal cancer specimens following total mesorectal excision (TME) surgery without neoadjuvant therapy.
Methods: Specimens from 60 patients who underwent TME were treated with modified fat clearing solution to retrieve LNs. The mesorectum was divided into right lateral, anterior, posterior and left lateral sides, which were further subdivided into three levels (upper, middle and lower).
Results: 1436 LNs were harvested, including 985 small LNs (<5 mm in size). The number of LNs from the anterior, posterior and bilateral mesorectum was 125 (8.7%), 696 (48.5%) and 615 (42.8%), respectively. In the longitudinal axis, the difference in distribution at the three levels was not significant. 200 LN metastases (mLNs) were detected in 33 patients. 48% (96/200) of these were small LNs. More mLNs, especially small LNs, were shown in the more advanced T stage patients. The mLN metastasis rate was not influenced by tumour level.
Conclusion: Small LN detection increased the accuracy of N staging by 20% in this study. The incidence of metastasis was the same among the anterior, bilateral and posterior areas of the mesorectum. An increased incidence of mLN metastasis in small LNs was associated with more advanced T staging. mLN metastasis rates in both middle and low rectal cancer were higher in the distal mesorectum than that in the proximal mesorectum. LN number and density were not consistent with spread of the primary tumour. Distal mLNs were found in 35% of cases of both middle and distal rectal cancer, implying a need for TME in both.