Breast cancer is categorized variously by histological type, receptor type and molecular phenotype.

Histological breast cancer types

Invasive breast cancer

Histology refers to the microscopic structure of tissues. Invasive, or infiltrating, breast carcinoma is characterized by cancer cells that have invaded healthy surrounding tissue. Invasive breast carcinoma, which form in epithelial tissue, is divided into five main histological types:

  • Ductal carcinoma develops in the milk ducts of the breasts up to 75% of cases
  • Lobular carcinoma forms in the cells that line the milk-producing glands (lobules) up to 15% of cases
  • Mucinous carcinoma tumor cells are poorly defined and produce mucus up to 2% of cases
  • Adenoid cystic carcinoma tumor cells resemble adenoid (glandular) and cystic cells extremely rare
  • Mixed tumors tumor cells are of different types in one tumor, e.g., ductal and lobular rare.
Ductal carcinoma has some less common subtypes:

  • Medullary carcinoma starts in the milk ducts, but tumor cells resemble the medulla (gray matter) of the brain up to 15% of all breast cancer cases
  • Inflammatory breast cancer starts in the milk ducts, but is characterized by involvement of the skin up to 3% of cases
  • Tubular carcinoma tumor cells have a tubular structure when viewed under the microscope up to 2% of cases
  • Piaget's disease of the nipple characterized by nipple involvement less than 1% of cases
  • Metaplastic carcinoma starts in the milk ducts, but tumor contains non-breast cells (e.g., skin, bone) extremely rare.

While most breast cancers are carcinomas, there are two types of breast sarcomas, which form in connective tissue:

  • Angiosarcoma forms in cells that line the blood vessels in the breast or underarm area extremely rare
  • Phyllodes tumor forms in the connective tissue of the breast, tumor has a leaf-shaped pattern of growth extremely rare.

Noninvasive breast cancer

Noninvasive breast cancer, which has not infiltrated healthy surrounding tissue and is not considered capable of spreading to other parts of the body, is divided into two main histological types:

  • Ductal carcinoma in situ (DCIS) ductal cancer cells form but remain inside the milk ducts of the breasts
  • Lobular carcinoma in situ (LCIS), also known as lobular neoplasia abnormal cells fill milk-producing glands (lobules).
Both DCIS and LCIS increase the possibility of invasive breast cancer, although DCIS is more likely to lead to it. The subsequent invasive breast cancer may be of a different type. For example, a woman with LCIS may go on to develop ductal breast cancer. Papillary carcinoma is a type of DCIS that rarely becomes invasive.

Most male breast cancer cases (up to 90%) are hormone receptor-positive ductal carcinomas. The remainder are mostly DCIS.

Receptor types

Breast cancer is also classified by cancer cell receptor status. Receptors are proteins found on some cells to which hormones such as estrogen and other compounds will attach. For example, estrogen receptor positive (ER+) cancer cells contain the estrogen-receptor protein. Breast cancer cells are categorized according to the following main types of receptors:

  • Estrogen receptor can be positive (ER+) or negative (ER-)
  • Progesterone receptor can be positive (PR+) or negative (PR-)
  • HER2/neu can be HER2/neu overexpressing (sometimes referred to as HER2 positive) or not.
Generally speaking, each of the main histological types can have any combination of receptor status. For example, lobular breast cancer can have ER-/PR- status, although lobular is more likely to be ER+/PR+ (also known as hormone receptor positive). Triple negative refers to breast cancer that is estrogen receptor negative (ER-), progesterone receptor negative (PR-), and does not overexpress HER2 (human epidermal growth factor receptor 2). There are four possible combinations of estrogen and progesterone receptor status: ER+/PR+, ER+/PR-, ER-/PR+ and ER-/PR-.

Estrogen receptors have been further categorized into two types: estrogen receptor-alpha and estrogen receptor-beta (ERα and ERβ). Some compounds that bind to ERα do not bind to estrogen ERβ and vice versa.

There are many other types of receptors that influence breast cancer and normally will not be included in routine breast cancer pathology but whose actions may be evaluated in breast cancer studies. Examples are vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF), and androgen receptors.

Molecular phenotypes: luminal, HER2 and basal breast cancer types

Researchers are increasingly categorizing breast cancer according to whether it is luminal, basal (also known as basal-like) or HER2 positive. These are molecular phenotypes, as determined by gene expression profiling. Based on comprehensive gene expression, breast cancer can be divided into luminal (expressing luminal cytokeratins 8 and 18), basal (characterized by cytokeratins 5 and 17), and HER positive (mostly, but not all, HER2 amplified). Luminal tumors can further be subdivided into luminal A and luminal B. It is hoped that these new classifications will help tailor breast cancer treatment, for example by identifying those most likely to benefit from a given type of chemotherapy.

In practice, these breast cancer types often are defined using the receptor subtype markers above, grouped in ways considered to be reasonable approximations of the molecular phenotypes. As such, there is not complete agreement concerning characteristics of these types. Below are the most common descriptions we have come across:

  • Luminal A - strong estrogen receptor positive (ER+) and progesterone receptor positive (PR+) and not HER2 overexpressing, with low tumor grade and low proliferation (Ki-67 < 15%). ER+/PR+/HER2- tumors are luminal A. The luminal A subtype represents up to 60% of all breast cancer patients.
  • Luminal B - weak to moderate ER+/PR+. Can be HER2- or HER2+. May have low tumor grade, but with higher proliferation and DNA instability than luminal A. ER+/PR- and ER-/PR+ tumors are normally luminal B
  • HER2 - HER2/neu overexpressing, i.e., HER2+
  • Basal or basal-like - most often triple negative, i.e., ER-/PR-/HER2-. However, not all basal tumors are triple negative and not all triple negative tumors are basal.