The generic definition of hyperplasia is an increase in cell number over normal. While this simplistic definition applies to hyperplastic disorders of the breast, the issue of hyperplasia of the breast is a very complex one, with sometimes confusing terminology. This discussion will be confined to explanations of the more commonly used terms and illustrations of the histologic features. There is also an attempt to summarize current thoughts on the clinical significance of the various entities. Hyperplasia of the breast is also classified this way.
Epithelial hyperplasia indicates a non-invasive proliferation of epithelial cells.
Both categories imply an abnormal proliferation, or increase, of the corresponding cell type. Both types also have a range of abnormal proliferation, from nearly normal to carcinoma in situ, and indicate important risk factors for developing invasive carcinoma. It is still confined within a basement membrane, implying that it is not yet invasive, and therefore without metastatic potential, or the ability to spread to parts of the body outside the breast. This is in contrast to infiltrating ductal carcinoma, which is invasive carcinoma.