Silhouette of a woman with breast area highlighted in red

How Does Breast Cancer Develop?

While the exact cause of breast cancer is still not fully understood, it is known that cancer begins when there are changes or mutations to the DNA in cells. These mutations may occur due to exposure to environmental factors, or they may be inherited, passed down from parents to children. Inherited mutations are often involved in breast cancers that run in families and may cause cancers to occur in younger individuals. However, most breast cancers are the result of acquired mutations, those that occur over time and may be the result of environmental or lifestyle influences.1


These mutations can cause breast cells to become cancerous. Cancerous, or malignant, cells multiply without restraint, and they can also break off from the original tumor site and spread through the body via the bloodstream or the lymphatic system. Some breast cancers also use substances that are normally healthy in the body to fuel their growth, such as estrogen, progesterone, or human epidermal growth factor receptor.2 When a breast cancer cell uses certain substances to grow and divide, the cancer cells have receptors on their surface in which the substances can attach and accelerate the growth of cancer. These cancers are called hormone receptor positive (estrogen receptor positive (ER+) or progesterone receptor positive (PR+) and/or human epidermal growth factor receptor 2 (HER2) positive (HER2+). These characteristics are important for determining treatment approaches.

Normal anatomy and function of the breast

To understand how breast cancer develops, as well as how it is characterized, it is helpful to understand the normal anatomy and function of breast tissue. The breast contains glands (also called lobes), ducts, and fat. After pregnancy, the lobes have the ability to make milk (for breastfeeding an infant), and the ducts carry the milk from the lobes to the nipple.3
While men do not breastfeed, they do have breast tissue, and breast cancer can occur in men.

Breast tissue also contains blood vessels and lymph vessels. Blood vessels carry nutrients and remove waste from the cells, and the lymph vessels carry lymph, a clear-to-white fluid that helps nourish tissues and carries foreign substances, like bacteria, viruses, cancerous cells, and damaged cells to the lymph nodes for removal. The lymph nodes are bean-shaped structures that are found along the lymphatic vessel pathways. Lymph nodes filter the lymph before it is returned to the bloodstream, and lymph nodes contain white blood cells, particularly lymphocytes and macrophages, which are key components of the immune system and help the body fight off infection and remove damaged cells, like cancer cells.3,4

Types of breast cancer

Most breast cancers originate from cells that form the ducts which carry milk to the nipple. These are called ductal carcinomas. Another common form of breast cancer are lobular carcinomas, which begin in the cells that form the glands that produce breast milk. Inflammatory breast cancer is a rare type of breast cancer that may not form a tumor that can be felt but causes the breast to look inflamed: red, warm, and swollen.3

Common genetic mutations in breast cancer

Approximately 5% to 10% of all breast cancers are caused by inherited genetic mutations. Two of these genetic mutations – BRCA1 and BRCA2 – are tumor suppressor gene mutations. Tumor suppressor genes normally manage cell growth and death and prevent the development of cancer. However, if tumor suppressor genes are mutated, cancer cells can multiply out of control. The BRCA1 and BRCA2 mutations confer an increased risk of breast cancer in women and men, as well as an increased risk of ovarian cancer in women. These genetic mutations are considered “high penetrance,” because they frequently lead to the development of cancer.1,3

Other genetic mutations that are associated with an increased risk of breast and/or ovarian cancer include ATM, PALB2, TP53, PTEN, CDH1, and STK11.5

When breast cancer spreads (metastasis)

Cancerous cells can spread into surrounding tissues or break off from the original tumor and spread throughout the body through the bloodstream or lymph system. Regional spread can occur to the lymph nodes under the arm (called axillary lymph nodes) or those above or below the collarbone (supraclavicular and infraclavicular lymph nodes). In certain cases, breast cancer can spread to the chest wall, invading the muscle and connecting tissues underneath the breast.

When breast cancer spreads to distant organs in the body (called metastatic breast cancer), it frequently spreads to the bones, brain, lungs, or liver. However, it can also potentially spread to other parts of the body. Despite where the cancer may spread, it is still considered breast cancer, as the cancer is named for where it originated.6

How breast cancer spreads

A recent study evaluated the patterns of how breast cancer spreads. Researchers began with the knowledge that the presence of breast cancer in the axillary lymph nodes are a risk factor for breast cancer to metastasize to other organs, but they didn’t know if the presence of the cancer in the axillary lymph nodes were responsible for cancer spreading or what routes cancer cells take.7,8

By looking at the DNA of the cancer cells and using a technique called next-generation sequencing, researchers were able to analyze the breast cancer cells from the initial tumor, the axillary lymph nodes, as well as other organs. Metastases often spread from the first distant organ to other organs. One of the most important findings was that the cancer cells in the axillary lymph nodes did not necessarily spread directly to other organs. Even if the presence of cancer in these lymph nodes was an indicator of the aggressiveness of the cancer, they were not necessarily the cause of further metastases. Rather, in certain instances, the tumor in the breast developed multiple distant metastases simultaneously.7,8

While this study was small (involving 99 samples from 20 patients with breast cancer), its findings are helping to uncover how breast cancer spreads and can help provide additional information for effective treatments.8

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Written by: Emily Downward | Last reviewed: December 2018