For more than a century, breast cancer has been classified by how the cells appear when looked at under the microscope. Early, non-invasive cancers are termed “in situ” because they have not yet invaded beyond the duct (tubes that transport milk) or lobule (where milk is made) tissue. While ductal carcinoma in situ (DCIS) is thought to be a precancerous lesion that should usually be surgically removed, lobular carcinoma in situ (LCIS) is most often not considered a precancerous lesion. Instead, it is considered to be a risk factor for the development of breast cancer.
Invasive breast cancer, or cancer that has invaded past the ducts or lobules into the surrounding tissue, accounts for the majority of breast cancer. Invasive breast cancer has been subclassified into multiple subtypes based on how the cells arrange themselves and appear under the microscope such as ductal (most common), lobular (second most common), medullary, tubular, and metaplastic, to name a few. While the “histological” subtype of cancer sometimes provides information regarding the behavior of the cancer (for example, tubular cancers are generally slow growing and do not metastasize), usually the histological subtype does not tell us what therapy will be most effective for a given cancer.
More recently, breast cancer has been subclassified by the presence or absence of several proteins on the cancer cells. These include the hormone receptors (estrogen receptor-ER, progesterone receptor-PR) and HER2 (Human Epidermal growth factor Receptor-2). The presence or absence of these receptors on the cell are very important in determining the best systemic treatment. At the time of biopsy and/or surgery the tumor tissue will be tested for these three receptors. Another important feature that is evaluated on the tumor tissue is the grade of the tumor. Tumor grade relates to how the individual cancer cells look under the microscope; cells that appear similar to normal breast cells and are not dividing quickly are low grade and cells that are unrecognizable as breast tissue and are dividing fast are more aggressive and high grade. The grade of the tumor provides useful information in deciding what medical treatment should be used.