| Molecular Subtype
|| Frequency/ Origin
|| Risk Groups
|| Pathology Features
| Luminal A
|| Most common subtype. Starts in the inner (luminal) cells that line the mammary ducts.
|| Patients more likely to be older compared to women with other subtypes.
|| Tumors are low-grade and have high levels of estrogen receptor.
|| Grows slowly. Less aggressive and less likely to recur.
|| Treated primarily with estrogen-lowering drugs and tamoxifen (endocrine therapy). Chemotherapy has a more limited role, particularly when the tumor doe not involve lymph nodes.
| Luminal B
|| Fairly common. Starts in the inner (luminal) cells that line the mammary ducts.
|| Patients often younger than those with Luminal A subtype.
|| Tumors are higher grade and have lower levels of estrogen receptor compared to Luminal A.
|| Grows more aggressively than Luminal A, with a significant relapse rate within the first five years after diagnosis.
|| Significantly worse prognosis than Luminal A. Endocrine therapy and chemotherapy are usually given, but tumors often exhibit resistance to both. New agents are under investigation, including drugs that target mutations in the PI3 kinase alpha gene.
|| Less common. Starts in the cells with features similar to those of the outer (basal) cells that line the mammary ducts.
|| Higher risk in women younger than 40. Patients are disproportionately African-American and women who have inherited a defective BRCA1 gene.
|| Tumors are high-grade and tend to be negative for estrogen receptor and HER2 protein (so called triple-negative breast cancer).
|| Grows quickly and behaves very aggressively, with a significant relapse rate within the first five years after diagnosis. Can appear as a large cancer despite a recent normal mammogram.
|| Treatment choices are focused on giving multiple cycles of chemotherapy regimens. A new class of agent called a PARP inhibitor may be effective in combination with chemotherapy.
| HER2 Overexpressing
|| Less common.
|| Higher risk in women younger than 40.
|| Tumors tent to be high-grade and produce excess HER2 protein and extra copies of the HER2 gene.
|| Grows quickly and behaves very aggressively if untreated. Can appear as a large cancer despite a recent normal mammogram.
|| Treatment has been transformed through the use of an antibody against HER2 called trastuzumab. Multiple other drugs have been developed to target HER2, many of which are active even when trastuzumab has stopped working.