What is breast cancer?

Breast cancer occurs when normal cells begin to multiply uncontrollably. They then turn into cancer cells. Sometimes cancer cells also spread outside the breast. This is called metastasis. Breast cancer does not occur only in women. In rare cases,mencan also develop it.

Many forms

Breast cancer is not one disease. There are different forms of breast cancer. They are classified based on where the cancer originates in the breast tissue, on certain characteristics of the cancer cells or on a heredity.  

Based on the site of origin in the breast

The most common form of breast cancer is ductal carcinoma, or breast cancer that develops in the milk ducts. Of these, there are two subforms: ductal carcinoma in situ (DCIS), which remains within the milk ducts and does not spread further and invasive ductal carcinoma, which does spread further into the surrounding tissue and is therefore more dangerous. The invasive ductal carcinoma is the most common, while DCIS is actually a precursor to breast cancer.

Another form of breast cancer originates in the mammary glands and is called lobular carcinoma. Of these, too, there is an in situ form that is limited to malignant cells in the mammary glands (pre-stage) and an invasive form that grows in surrounding tissue and can spread.  

Based on properties of cancer cells

Most breast cancers are hormone-sensitive. That means the cancer cells carry receptors for female hormones and grow under the influence of these hormones (estrogen and/or progesterone). Receptors are like locks on the surface of cells where hormones fit like keys. Thanks to these receptors, hormone-sensitive breast cancers are easily treatable with anti-hormone therapy. Other breast cancers are not hormone-sensitive at all and are called triple-negative breast cancer. In this case, the cancer cells have no receptors for either female hormone and also no receptor for the HER2 protein ("triple-negative" or three times negative). Triple-negative breast cancer is more common in younger women, and requires a different type of treatment. There is also a form of breast cancer with many HER2 receptors on the cancer cells: the HER2-positive breast cancer grows faster, but also requires targeted treatment.

Based on the cause

The cause of breast cancer is usually unknown unless it is a fault in the hereditary material. About 10% of all breast cancers are hereditary. In these cases, the cause is a breast cancer gene that was inherited from one or both parents. The two best known breast cancer genes are BRCA1 and BRCA2. They significantly increase the risk of breast cancer. Carriers of BRCA1 have a 55 to 70% risk of developing breast cancer and also have a higher risk of ovarian cancer. Carriers of BRCA2 have 45 to 60% risk of developing breast cancer and are also at greater risk of ovarian cancer.  

There are at least 10 other genes that increase the risk of breast cancer, but to a lesser extent than BRCA. They are less frequent. CHEK2 is a well-known example and increases the risk by 20 to 40%.

Rare forms of breast cancer

There is a whole slew of rare breast cancers that are difficult to categorize into mentioned categories, including inflammatory breast cancer, Paget's disease, mucinous carcinoma, metaplastic carcinoma, tubular carcinoma and medullary carcinoma.