Stage 4 metastatic breast cancer (MBC) has spread beyond the breast tissue and surrounding lymph nodes. MBC may spread, or metastasize, to the brain, bones, lung, liver, or other parts of the body.

MBC refers to cancer that begins in breast tissue before spreading to other areas of the body. When cancer metastasizes, or spreads, it travels to another organ.

Brain metastases are cancer cells that have spread to the brain. When MBC reaches the brain, medical professionals still classify the brain metastases as breast cancer rather than brain cancer.

This article takes a look at what happens when MBC spreads to the brain.

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Scientists are not entirely sure why breast cancer metastasizes to the brain. Research suggests that it can occur when breast cancer cells enter the circulatory system.

Some breast cancer cells have or develop the ability to grow and spread in the brain after they cross the blood-brain barrier. When these cells adapt to the environment of the brain, they can multiply and result in brain metastases.

About 1 in 8 women will receive a diagnosis of invasive breast cancer during their lifetimes, the American Cancer Society reports. “Invasive” means that cancer cells have grown through the lining of ducts in the breast and into the surrounding tissue.

By contrast, roughly 1.2 in 100,000 men will get a diagnosis of any type of breast cancer in their lifetimes.

Most people with breast cancer receive the diagnosis before their cancer has traveled to distant parts of the body. However, nearly 30% of women who have a diagnosis of early stage breast cancer go on to develop MBC.

MBC metastasizes to the brain in about 10–15% of cases, according to the advocacy organization MBC may also spread to other parts of the body, such as the bones, lungs, or liver.

Certain types of breast cancer are more likely than others to spread to the brain.

These include more aggressive types, such as cancers that test positive for excess human epidermal growth factor receptor 2 (HER2), a protein that promotes the growth and spread of cancer cells.

Triple-negative breast cancers are also more aggressive and more likely to spread to the brain. They test negative for progesterone receptors, estrogen receptors, and excess HER2 protein.

Symptoms of brain metastases can vary from person to person. They may include:

In some cases, brain metastases cut off blood flow in the brain. This results in a stroke, which may cause sudden symptoms such as:

  • weakness or numbness on one side of the face, arm, or leg
  • confusion
  • difficulty speaking or understanding speech
  • vision problems in one or both eyes
  • loss of balance or coordination
  • difficulty walking
  • severe headache

When a doctor diagnoses MBC, they may recommend aggressive treatment with one or more of the following:

In some cases, they may recommend surgery to remove a tumor.

They may also prescribe pain relief medications or other treatments to help ease symptoms and any side effects.

Doctors consider many factors, including the following, when developing a treatment plan:

  • the type of breast cancer
  • the number, location, and size of tumors
  • how the cancer has responded to past treatments
  • the person’s age and overall health
  • their treatment goals
  • their symptoms

The aims of treatment typically include:

  • shrinking or slowing the growth of tumors
  • reducing symptoms
  • prolonging life
  • improving the quality of life

When breast cancer spreads to the brain, it is not curable. However, treatment may help increase life expectancy and improve the quality of life.

The 5-year survival rate for MBC is about 31%, according to the American Cancer Society. This means that about 31% of people with MBC are still living 5 years after they received the diagnosis.

It is worth remembering that all survival rates derive from averages of past data, and they may not take into account recent advances in treatment.

Additionally, factors specific to each person play an important role. Anyone with MBC needs to speak with their doctor to learn about their treatment options and outlook.

If metastatic breast cancer (MBC) spreads to the brain, it can cause symptoms such as slurred speech and changes in mood or behavior.

Treatments can help shrink brain tumors, slow their growth, and improve the quality of life. Meanwhile, researchers continue to develop new treatments to improve survival rates.