Pathology of minimal metastatic disease in sentinel lymph nodes in breast cancer

Sentinel lymph node biopsy (SLNB) is now widely used as an alternative to axillary lymph node dissection to stage the axilla in women with early-stage breast cancer. The detailed pathological work-up of sentinel nodes has presented new challenges in the field of pathology. For example, many patterns of low volume disease - isolated tumour cells (ITCs) and micrometastasis (MI) - are difficult to classify and current classification systems are only moderately reproducible. The significance of ITCs and MI in terms of predicting non-SLN involvement and overall outcome is also unclear. The optimal methodology for evaluating the SLN is also unresolved. In this review, we give an overview of these controversial areas in SLN pathology, including the classification, reproducibility and clinical significance of low volume disease; pathological evaluation of the SLN; and methods of intraoperative assessment.