Axillary sentinel lymph node micrometastases with extracapsular extension: a distinct pattern of breast cancer metastasis?

Aims: To examine the frequency of extracapsular extension (ECE) of sentinel lymph node (SLN) metastases in breast cancer according to metastasis size, and to characterise ECE in micometastases. Methods: If initially negative, SLNs were examined by step-sectioning and immunohistochemistry. Non-SLNs were not subjected to enhanced pathology. Positive axillary SLNs were analysed for metastasis size and the presence of ECE. Results: Of 885 successful SLN biopsy cases, 343 (39%) exhibited SLN involvement, and 115 (34%) displayed ECE. Of the latter, 107 underwent axillary dissection, and 63 (59%) of these demonstrated non-SLN metastases. The presence of ECE correlated with metastasis size (coefficient 0.92). Eight (10%) of the 84 micrometastatic SLN cases had ECE, and two of these were associated with non-SLN metastases. Only ECE and only the intraparenchymal nodal part of the micrometastasis were seen in some sections of five cases each. The primary tumours of the micrometastatic cases with ECE were non-high-grade and often of tubular type. Conclusions: The frequency of ECE increases with increasing nodal metastasis size. Minimal nodal metastases with ECE may represent a distinct pattern of nodal involvement with a predominant capsular and extracapsular, but only minimal or no nodal parenchymal component, predominantly seen in non-poorly differentiated and/or tubular carcinomas. This presentation of nodal metastasis can sometimes pose differential diagnostic problems, and should be distinguished from massive metastases presenting with ECE because it does not seem to be so commonly associated with non-SLN metastases or a massive metastatic load to the axilla as ECE of SLN metastases in general.