Breast cancer occurs when cells in the breast become abnormal and grow out of control. HER2-positive breast cancers are those in which higher than normal levels of a growth-promoting protein known as HER2 are present.
HER2-positive breast cancers usually grow and spread faster than other breast cancers, but they tend to respond to specific drugs targeting the HER2 protein.
The exact cause of HER2-positive breast cancer is unknown, but it is believed environment, lifestyle, and genetics may be factors.
Risk factors for developing breast cancer in general include:
- Being a woman
- Inherited genes, including BRCA1 and BRCA2 and others
- Family history or personal history of breast cancer
- Age over 55
- Physical inactivity
- Overweight/obesity
- Alcohol use
- Use of hormonal birth control
- Hormone replacement therapy (HRT) for menopause, particularly estrogen and progesterone (combined hormone therapy)
- Having a first child after age 30 or having no children
- Dense breast tissue
- Ethnicity: White women are slightly more likely to develop breast cancer during their lifetime, but African-American women are more likely to develop breast cancer under age 45
- Certain benign breast conditions
- Early onset menstruation (before age 12)
- Menopause after age 55
- Radiation to the chest
What Are Symptoms of HER2-Positive Breast Cancer?
The American Cancer Society’s warning signs and symptoms of breast cancer include:
- Breast lump
- Lump in the underarm area (armpit)
- Swelling or thickening of the breast
- Irritation or dimpling of the breast skin
- Pain in any area of the breast
- Nipple pain
- Nipple turning inward
- Changes in the size or shape of the breast
- Skin changes on the breast: redness, scaliness, flaky skin, or thickening of the nipple or breast skin
- Nipple discharge that is not breast milk, including blood
How Is HER2-Positive Breast Cancer Diagnosed?
In some cases, a woman may discover a lump or may notice changes in the breast. A doctor will perform a physical exam to look for breast changes such as:
- A lump in the breast
- Changes in the size or shape of the breasts
- Dimpling skin on the breast
- Pulling in of a nipple
- Discoloration of breast skin
Tests used to confirm a diagnosis of breast cancer include:
- Mammogram (a special type of X-ray)
- 3D tomosynthesis is a special new type of digital mammogram
- Breast ultrasound
- Breast magnetic resonance imaging (MRI)
- Not usually used to screen for breast cancer but may be used in the following situations:
- Screening young women, especially those with dense breasts, who have an increased risk of breast cancer (e.g., mutations in the genes BRCA1 or BRCA2)
- Screening for breast cancer in women diagnosed with cancer of the lymph nodes (glands)
- Screening of women with newly diagnosed breast cancer with extremely dense breasts on mammograms
- Not usually used to screen for breast cancer but may be used in the following situations:
- Biopsy, in which samples of tissue from the breast are removed and examined
Tests used to determine if breast cancer is HER2 positive include:
- IHC test (ImmunoHistoChemistry)
- FISH test (Fluorescence In Situ Hybridization)
What Is the Treatment for HER2-Positive Breast Cancer?
Treatment for HER2 positive breast cancer usually involves anti-HER2 therapies (also called HER2 inhibitors or HER2-targeted therapies) that specifically target HER2 receptors. These drugs only work on HER2-postive breast cancer.
Anti-HER2 therapies include:
- Monoclonal antibodies
- Trastuzumab (Herceptin, Herceptin Hylecta), which can be given with chemotherapy and sometimes another targeted therapy called pertuzumab (Perjeta)
- Herzuma, Kanjinti, Ogivri, Ontruzant, and Trazimera are biosimilars to Herceptin
- Margetuximab-cmkb (Margenza) used in combination with chemotherapy
- Pertuzumab (Perjeta) used in combination with Herceptin and chemotherapy
- Antibody-drug conjugates
- Am-trastuzumab-deruxtecan-nxki (Enhertu)
- Used to treat people diagnosed with metastatic HER2-positive breast cancer previously treated with at least two other anti-HER2 medicines
- Also used to treat HER2-positive breast cancer unable to be removed with surgery
- T-DM1 or ado-trastuzumab emtansine (Kadcyla)
- Used to treat people diagnosed with HER2-positive metastatic breast cancer that has previously been treated with Herceptin and taxane chemotherapy
- Can be used after surgery in people diagnosed with early-stage HER2-positive breast cancer treated with Herceptin and taxane chemotherapy before surgery and had residual disease found during surgery
- Pertuzumab, trastuzumab, and hyaluronidase-zzxf (Phesgo)
- Used in combination with chemotherapy before surgery for early-stage or inflammatory HER2-positive breast cancer
- Also used after surgery for early-stage HER2-positive breast cancer with a high risk of recurrence
- Am-trastuzumab-deruxtecan-nxki (Enhertu)
- Pan-HER inhibitor
- Neratinib (Nerlynx)
- Used to treat early-stage HER2-positive breast cancer for an extended period of time after surgery and chemotherapy
- Also used in combination with chemotherapy to treat advanced-stage and metastatic HER2-positive breast cancer
- Neratinib (Nerlynx)
- Signal transduction inhibitor
- Lapatinib (Tykerb)
- Used to treat advanced or metastatic breast cancer that is HER2-positive in combination with chemotherapy medicines
- Lapatinib (Tykerb)
- Tyrosine kinase inhibitor
- Tucatinib (Tukysa)
- Used to treat metastatic or locally advanced HER2-positive breast cancer that can’t be completely removed with surgery, after the cancer has been treated with at least one anti-HER2 medicine.
- Tucatinib (Tukysa)
HER2-positive breast cancer may also be treated with chemotherapy, surgery, radiation, and other treatments.
From 
https://www.uptodate.com/contents/breast-cancer-guide-to-diagnosis-and-treatment-beyond-the-basics
http://www.cancer.org/cancer/breastcancer/overviewguide/breast-cancer-overview-what-causes
https://www.breastcancer.org/