Discussion

There have been several recommendations regarding the minimum number of LNs to be assessed for the reliable staging of CRCs1 ranging from 612 to 20,13 although it is mostly accepted that there is no such a minimum, and all LNs need to be recovered from resection specimens and evaluated by histology. The staging accuracy increases with the number of LNs assessed, and such an improvement in staging reliability is also associated with improved survival.14 15 In the setting recommending the examination of all LNs resected, the suggested values for the number of LNs to be assessed are better understood as numbers reflecting the accuracy of staging. These numerical recommendations never took into account the fact that LNs are not equivalent in terms of target for metastatic spread, and qualitative features may influence the accuracy of staging, even when fewer than all LNs resected are examined histologically.

Several qualitative features of the LNs enabling a selection between metastatic and metastasis-free ones have been assessed, including their size,7 their localisation9 and their direct connection with the primary tumour site assessed by lymphatic mapping.8 16 17 On the basis of a preliminary study, based on the analysis of 100 CRC cases, we have proposed that metastatic LNs (at least the first ones) are more likely to be located in the close vicinity of the tumour. It was concluded that this close vicinity was the mesocolon/mesorectum beneath the length of resected bowel including the tumour and the 3 cm sidelong part proximal and/or distal to it. Our present data strengthen this conclusion, as they make it clear that analysing the LNs in this part of the pericolic and perirectal fat (segment AB) is sufficient for the accurate staging of CRCs as node-positive or negative in nearly all cases, and it also allows an accurate pN0 versus pN1 versus pN2 classification in most cases. Whenever LNs in segment AB are involved, the chances of having further metastatic LNs in the rest of the nodes are increased. Indeed, 61 of the 365 CRCs (16.7%) with positive AB nodes had nodal involvement in CD LNs, too. Adding some further precaution to avoid misclassification such as the examination of CD LNs when there are three AB nodes involved or when the number of AB LNs is low (fewer than seven in our original recommendation, or fewer than 12 to comply with most recommendations) should even improve the staging accuracy with limited workload.

Besides our initial report,9 we are aware of only one study which assessed the localisation of LNs in a fashion similar to ours. Pusztaszeri et al, in a recent study of 345 CRCs, used a cut-off length of 5 cm in both directions from the tumour and concluded that metastases seldom occur in the distant nodes only.18 They reported a just lower median number of LNs overall (n=17) and noted only six cases which were devoid of metastatic LNs in the close fraction but had positive nodes in the distant fraction. Although their accuracy rate for a qualitative nodal status was somewhat below ours (98.3%), this is not a major difference. Most of their false-negative statuses (five out of six) were in rectal cancers where they recommended the examination of all LNs. Our findings do not fully match this distribution, but half of the false-negative cases (2/4) and most (10/14) of the understaged tumours were in the rectum, a fact that may support a more intensive search for LNs if the primary cancer is in this region.

Although it has been suggested that sentinel LNs of CRCs may be located away from the primary tumour and can be found even at sites falling outside their traditional resection margins (eg, at the splenic flexure for a carcinoma in the ascendent colon),19 20 this is a rare phenomenon (Sukamal Saha, oral communication at the 5th East–West Colorectal Days, Hajduszoboszlo, 10–12 September 2009). The rarity of such distant nodal metastases is also supported by the present data. It must also be considered that the pooled false-negative rate of sentinel LN examination in CRCs is around 9%,17 which is much higher than the false-negative rate of assessing only the LNs close to the primary tumour as defined in the present study.

It is suggested that sufficiently accurate nodal staging can be reached by the limited and stepwise protocol we suggest (ie, the assessment of relatively distant LNs only if necessary either because of a higher suspicion for a higher nodal category or because of too few LNs recovered during the limited work-up). This may spare time and reduce workload in case of long resected segments such as hemicolectomy specimens. As most LNs are recovered from the area we propose to investigate first, the number of nodes assessed should also not diminish too much with the recommended approach. However, the present results should not be used to substantiate substandard LN recovery rates potentially leading to understaging, and all LNs in the suggested area should be thoroughly looked for.