![]() 2 The occult metastasis investigation was initially applied as a ‘proof of principle’ but was also viewed as a mechanism to more accurately stage breast cancer patients, reopening a Pandora's box from the 1940s. Sentinel nodes were more likely to contain metastases, if they were present, and occult metastases deeper in paraffin blocks were more likely to be identified in SLNs than non-SLNs. The sentinel node biopsy technique for breast cancer rapidly diffused throughout the surgical community. This new technique complemented sentinel node biopsy research using vital blue dyes that was being investigated particularly in melanoma but also in breast cancer. 1 This made it possible to identify the location of the sentinel node before skin incision and use the probe to guide surgery. In 1993 a pilot series of sentinel lymph node (SLN) biopsies in breast cancer patients was published where the SLN was identified using a hand held gamma probe after injection of a radioisotope tracer around the breast tumor. If we ultimately adopt more comprehensive microscopic evaluation of SLNs, the candidate sampling strategies need to be carefully considered in the context of statistically valid sampling strategies. It is critical that both clinicians and pathologists understand there is a random component to micrometastasis distribution within the three-dimensional paraffin tissue blocks. In the context of the new molecular classification of breast cancer, subgroups may be identified where detection of micrometastases has clinical significance. The prognostic significance of these missed micrometastases is still being evaluated as we await SLN outcome studies. A single section from blocks prepared in this manner will identify all macrometastases present but smaller metastases will be missed. The most important aspect of the sentinel node examination is careful attention to slicing the SLN no thicker than 2.0 mm and correct embedding of the slices to assure we identify all macrometastases larger than 2.0 mm. What is needed is adherence to a standardized evaluation protocol. Despite recommendations from the College of American Pathologists and the American Society of Clinical Oncology, heterogeneity in the approach to SLN evaluation exists. It is quite clear that the more sections we evaluate from SLNs the more metastases we identify however, it is impractical to expect the practicing pathologist to mount, stain, and microscopically examine every section through the SLN paraffin blocks. As a result, our nodal classification and cancer staging have evolved to recognize the continuum of nodal tumor burden rather than a simplistic dichotomous stratification. National data show that many women previously classified node negative are now classified minimally node positive. The limited number of SLNs compared with an axillary dissection has prompted more comprehensive lymph node analysis increasing detection of micrometastases. Sentinel lymph nodes (SLN) are more likely to contain metastatic breast carcinoma than non-SLNs.
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