Cytokeratin IHC of unselected axillary lymph nodes from patients with ILC has been shown to upstage these patients more often than those with ductal carcinoma,20,23
and some laboratories have therefore introduced this method as a routine means of evaluating all lymph nodes removed in this subset of patients.27
As sentinel lymph node biopsy selects the nodes which are the most likely sites of regional metastases, it would be wiser to limit the use of IHC to these nodes. Most guidelines do not recommend IHC for the evaluation of sentinel lymph nodes in general practice,6,10,13,14,16
but it may be used in special cases, such as cases of ILC.6
As sentinel node biopsy is as reliable in ILC as in ductal cancers,28,29
we evaluated the role of cytokeratin IHC in a multi‐institutional cohort of ILC patients who underwent sentinel lymph node biopsy.
Our results show that tumour size influences the nodal involvement in lobular carcinoma. This finding is consistent with nodal involvement being more common in larger tumours of any histological type. Although no comparison was made between the rate of nodal involvement in different types of tumour, our results are in agreement with data from the era before sentinel node biopsy. We found that 34% of all cases of nodal involvement were detected by IHC, which is higher than the rate reported for breast carcinoma in general or of ductal carcinoma, and is in the rate range reported for ILC.18,19,20,21,22,29
Although there were institutional methodological variations in this retrospective study, and also differences in the rate of nodal involvement and IHC positive cases, these latter differences can only partly be accounted for by the methodology. Mean tumour sizes were also different (table 1), and this may have altered the metastatic rates mentioned above. Obviously, the more detailed the histological protocol, the more positive cases will be detected.30,31
Diversity of methods may be a problem with this study, but such variations in methodology can be found when comparing the various studies cited previously,18,19,20,21,22
and are encountered in nearly all reviews dealing with the upstaging role of IHC in breast cancer.32
Owing to the large number of cases, it is felt that the conclusions below can be relied upon, even if there may be some variation in the pathological approach to the reported lymph nodes.
To the best of our knowledge, this is the first report on the differential rates of isolated tumour cells, micrometastases, and larger metastases detected by IHC. Surprisingly, despite the fact that the isolated tumour cell category of nodal involvement was the one in which IHC produced the greatest increase in detection rate (17/19; 0.89), the largest category of IHC‐detected nodal involvement (n
40) was the micrometastases. This is probably because ILC often produces involvement of the nodal parenchyma,33
and we considered this to represent micrometastasis.25,26
Obviously, a smaller number of macrometastases was also first detected by IHC.
The displacement and passive transport of tumour cells after needling procedures and excision biopsy34,35,36
have been proposed as a mechanism for the lodging of tumour cells in regional lymph nodes, especially in sentinel lymph nodes from breast cancer patients. The phenomenon of artefactual tumour cell seeding has been seen in cases of ductal carcinoma in situ37
and, although this event may be rarer in the less cellular but less cohesive lobular carcinomas, isolated intrasinusoidal epithelial cells may well be of this origin. It has been postulated that these cells are detectable mainly by IHC only, and therefore that the IHC detected cells are irrelevant. Patients undergoing prophylactic mastectomies after biopsies were only rarely found to have IHC detected epithelial cells in their sentinel nodes, however.38
This suggests that cancer needs to be present for there to be an increased rate of IHC positive sentinel nodes after biopsy or other means of physical manipulation of the tumour. These diagnostic or therapeutic procedures alone cannot be responsible for all cases of IHC detected nodal involvement.
A word of caution is required, as cytokeratin positive nodal structures cannot always be equated with metastatic nodal involvement. Besides the artefactually displaced tumour cells discussed above, normal constituents of the lymph nodes may also stain with anti‐cytokeratin antibodies. Interstitial reticulum cells have been reported to be cytokeratin positive, especially when stained by CAM5.2 or an in‐house cytokeratin cocktail, whereas this was much rarer or absent with AE1/AE3.39,40
Plasma cells have also been reported to stain with CAM5.2 and pan‐cytokeratin.40
Rarely, occasional cells compatible with histiocyte morphology also stain weakly with cytokeratin antibodies. Obviously, rare epithelial inclusions of the lymph nodes are also cytokeratin positive.41
Morphology should therefore never be neglected in the face of positive cytokeratin staining, and this will usually help to discriminate cancer cells from the others. Whenever there is doubt as to the nature of cytokeratin positive cells, these should not be called metastases, in line with the general rules of the TNM staging of cancers.42
None of the cytokeratin positive cells in this study was considered to represent inclusions or non‐epithelial cells.
Although the rate of nodal involvement increased with increasing tumour size, and this was also true for the macrometastases; the rate of nodal involvement, isolated tumour cells, and micrometastases detected by IHC tended to decrease with increasing tumour size (tables 2 and 3). The lack of an association between IHC detected sentinel node involvement and predictors of HE detected sentinel node involvement was reported earlier.37
Although this may be because isolated tumour cells are commonly (although certainly not always) the result not of a true metastatic process but rather of previous procedures and manipulations,37
another possible explanation could be that IHC tends to detect less obvious nodal involvement (generally falling into the category of isolated tumour cells or micrometastases), which is more common with smaller tumours, whereas larger tumours have already established larger metastases that are more likely to be detected by HE staining even in ILC.
Although we were unable to analyse the prognostic significance of these metastases in terms of relapse or survival, we did analyse the status of further lymph nodes in the axilla in the 161 patients who had an axillary dissection after the diagnosis of sentinel node involvement of any type. Twelve of 50 IHC detected sentinel lymph node metastases from ILC, but none belonging in the isolated tumour cell category, were associated with non‐sentinel node involvement, which was higher than our previous meta‐analysis (around 9%) would have suggested for IHC detected sentinel node involvement in breast cancers in general.32
None of the studies included in that meta‐analysis drew conclusions in relation to the histological type of tumour.
Take home message
Immunohistochemistry is recommended for the evaluation of sentinel nodes from patients with lobular breast carcinoma, as the micrometastases or larger metastases demonstrated by this method are often associated with a further metastatic nodal load.
Our findings suggest that sentinel lymph nodes should be investigated by IHC if the primary tumour is of lobular type, because this approach may often detect micrometastases and even larger metastases, requiring further axillary treatment by current standards.