Silhouette of a woman with breast area highlighted in red

How Does Breast Cancer Develop?

Reviewed by: HU Medical Review Board | Last reviewed: January 2022

Metastatic breast cancer can feel like a scary diagnosis. You may be wondering how you ended up with breast cancer and how it metastasized, or spread. You may be wondering how it is treated. These are important questions. To start, it might be helpful to understand how cancer develops.

What is cancer?

Cancer is an error in the DNA of the cells that causes them to grow out of control. This usually causes a growth that is called a tumor. Tumors that do not spread to other parts of the body are called benign and are not cancer. Tumors that can spread to other parts of the body are called malignant. Not all cancers have tumors.1

What are the common types of breast cancer?

It is important to understand that there is not just 1 type of breast cancer. Breast cancer is a category for multiple cancers that start in the breast.

The most common breast cancer is ductal carcinoma. This cancer starts in the milk ducts of the breast, just like its name suggests. The other more common breast cancer is lobular carcinoma. It starts in the lobules of the breast. Lobules are small ducts that produce milk.1

There are a few less common breast cancers. These include:1

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What are the subtypes of breast cancer?

Breast cancer is classified into subtypes. Doctors test for these subtypes when someone is diagnosed to help them decide the best treatment.

There are 3 subtypes of breast cancer:1-3

  • HR-positive – Hormone receptor-positive cancer (HR-positive) is a type of cancer that generally needs estrogen (ER-positive), progesterone (PR-positive), or both to grow. This is the most common subtype of breast cancer. It tends to occur more in people who are beyond menopause, though it can happen to people of any age. Hormone receptor-negative means the cancer does not have ER or PR.
  • HER2-positive – This cancer has high levels of a gene called human epidermal growth factor 2 (HER2) or high amounts of the HER2 protein. HER2-positive cancers tend to grow quickly. They can be HR-positive or negative as well.
  • Triple-Negative – If the cancer is not positive for ER, PR, or HER2, it is called triple-negative. This subtype of cancer is more common in younger women. It is also more likely to be associated with the BRCA gene mutation.

What is metastatic cancer?

Tumors usually start in a certain area of the body. For example, this can be the lungs, liver, or breast tissue. If the tumor has not spread past the original tissue and surrounding lymph nodes, it is considered localized or locally advanced.2

If cancer has spread past the original tissue to other parts of the body, it is called metastatic cancer.2

Metastatic cancer

Metastatic cancer, or advanced cancer, has spread to 1 or more parts of the body.2

Advanced breast cancer can spread to any part of the body. It usually spreads to the liver, brain, lungs, and bones. Breast cancer that has spread to different sites is still considered breast cancer. This is because the cancer cells started in the breast.2,3

Cancer spreads when cells from the original tumor break off and enter the bloodstream or lymph system. These cells then grow in other parts of the body. Usually, your body tries to attack foreign cells it finds. Your body may not attack the cancer cells in other parts of your body because they are similar to your own cells.1-3

Metastatic breast cancer may be a result of cancer recurrence. A cancer recurrence happens when you have been in remission and you are diagnosed with cancer again. If the diagnosis happens in a different part of the body than the original cancer site, it is still metastatic cancer. This means that some cancer cells were not completely killed off during treatment and have moved to other parts of the body.3

Some people find out they have metastatic breast cancer when they are first diagnosed. This means that the breast cancer has already spread to other parts of the body by the time it was discovered.2,3

If you have any questions about your diagnosis, be sure to reach out to your healthcare team. They can explain your breast cancer type and subtype, work with you to find the best treatment, and help you maintain the best quality of life through your treatment and beyond.