Breast cancer subtypes
In every case of invasive breast cancer, pathologists look for three molecular markers in the tumor sample to determine the breast cancer type and develop a treatment plan. These markers are: the estrogen receptor (ER), progesterone receptor (PR), and HER2. The results of these tumor marker tests play a major role in choice of systemic therapy:
- ER-positive and PR-positive breast cancers: these are generally are treated with hormone therapy. Chemotherapy may also be recommended depending on the stage of the tumor and other factors. An agent known as a CDK4/6 inhibitor (Ibrance) has been FDA approved for treating metastatic or advanced hormone receptor positive, Her2-negative breast cancer in combination with an aromatase inhibitor.
- HER2-positive breast cancers: these cancers benefit from therapies that target the Her2 molecule. Therapies include trastuzumab (Herceptin®) and pertuzumab (Perjeta®). Anti-HER2 therapy is always given with chemotherapy in patients with early-stage, curable tumors. Patients whose tumors are HER2-positive and ER-positive receive hormone therapy, as well as chemotherapy and anti-HER2 therapy (trastuzumab or pertuzumab)
- ER-negative and PR-negative breast cancers: these are almost always are treated with chemotherapy because hormone therapy does not benefit patients with these types of tumors.
- Triple-negative breast cancers: tumors that are negative for all three markers are referred to as triple-negative breast cancer or sometimes "TNBC" for short. Like other ER-negative breast cancers, triple-negative breast cancers are almost always treated with chemotherapy. People with BRCA1 mutations are more likely to develop triple-negative breast cancer than any other type of breast cancer.
Additional tests can be performed on tumor samples to help guide treatment. Prognostic genomic tests can also help guide treatment decisions. For some patients with advanced cancers, tumor testing can look for additional tumor markers to help guide the choice of targeted therapies. At the moment, this type of tumor testing for targeted therapy is mostly experimental and used within research studies.