When breast cancer metastasizes, it most commonly spreads to the bones—here's what that means, and which treatment options are most commonly used in this situation.

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Breast cancer impacts millions of people every year. But metastatic breast cancer—which is breast cancer that has spread beyond the breast—is somewhat less common. Only about 30% people who have breast cancer later develop metastatic disease

While breast cancer has the potential to spread anywhere, "the most common area for breast cancer to metastasize to is the bones," Dean Tsarwhas, MD, head of the oncology program at Northwestern Medicine Lake Forest Hospital, tells Health. In fact, 70% of breast cancer patients who later develop stage 4 cancer have bone metastases, according to 2017 data published in ecancermedicalscience.

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Credit: GettyImages / AdobeStock

But what are the implications of metastatic breast cancer in the bones, and how does that affect a patient's treatment and prognosis? Here's everything you need to know, according to top oncologists. 

What does it mean when breast cancer metastasizes to bones?

As mentioned earlier, metastatic breast cancer is when cancer has spread beyond the breast and the surrounding lymph nodes to other parts of the body. This can happen either because a cancer has come back after treatment (called recurrent cancer) or because a cancer did not respond to treatment or wasn't treated in time, allowing it to spread. In the case of metastatic breast cancer in the bones, cancer cells take up residence in parts of a person's bone, causing lesions that replace healthy tissue with cancer cells.  

Modern science still doesn't quite understand exactly how cancers metastasize, Nancy Lin, MD, a medical oncologist who specializes in breast cancer treatment at Dana-Farber Cancer Institute in Boston, Massachusetts, tells Health. "We know that at some point the cancer manages to find its way into the bloodstream and then through that is able to get to other parts of the body," she says. Beyond that, it's unclear why certain cancers tend to spread to specific spots in the body like the bones, but there are some theories. 

For example, metastatic estrogen-receptor positive breast cancer often spreads to the bones, says Evelyn Toyin Taiwo, MD, hematologist and oncologist at Weill Cornell Medicine and NewYork-Presbyterian Brooklyn Methodist Hospital. This could be because your bones have estrogen receptors, she theorizes, which might provide a favorable environment for these rogue cancer cells to grow. But there is still a lot of research that needs to be done to better understand this association. 

Any part of your skeletal system can be impacted by metastatic cancer, but per the American Cancer Society (ACS), the spine is the most common site of bone metastasis. Other areas include the hip, femur, upper arm, ribs, and skull.

What are the symptoms of bone metastasis in breast cancer?

All three doctors say that new, progressive pain in your bones or joints is the most common symptom of metastatic breast cancer in bones. "I always tell patients to inform me if there's pain that's not getting better," says Dr. Tsarwhas. This can sometimes be confused with arthritis or other pre-existing chronic pain issues, he says, which is why it's important for breast cancer patients to be proactive about any new pain they encounter.

New fractures or unexplained fractures can also be a sign of bone metastasis, Dr. Tsarwhas adds. Cancer can weaken bones and make them break more easily. "New lumps or bumps in the lymph node area...could be a sign of recurrent breast cancer as well," he says.

A person with metastatic breast cancer in their bones may also experience more general cancer symptoms, such as fatigue, lack of appetite, and extreme, unexplained weight loss

How is metastatic breast cancer in bones treated?

Metastatic breast cancer in the bones is typically not curable. Instead, Dr. Taiwo says that doctors prioritize slowing the cancer's growth and managing related symptoms. "The focus really is trying to extend life and maintain quality of life," she says.

Metastatic breast cancer patients often receive traditional medications like chemotherapy, immunotherapy, and targeted treatments for their specific cancer type (say, hormonal treatments for estrogen receptor-positive breast cancer), says Dr. Tsarwhas. But there are also many bone-strengthening medications used to help people with bone metastases. Bone-targeted agents like denosumab, for example, are used to help slow bone degradation and reduce the risk of fractures, he says. (Many of these treatments, adds Dr. Lin, are similar to what people with osteoporosis take.)

Localized radiation treatment can also help shrink bone cancer cells and reduce pain, adds Dr. Taiwo. This can hugely improve a patient's quality of life, she says, as bone lesions can be incredibly painful. 

"We also look for any bones that might have the potential to cause a problem in terms of a fracture," says Dr. Lin. In those cases, she says doctors often send people to orthopedic surgeons for preventative surgeries to help stabilize bones. For example, surgeons can use medical-grade bone cement to seal and support existing fractures, or recommend joint-replacement surgery to ensure a person has pain-free mobility

What’s the life expectancy with metastatic breast cancer in bones?

Metastatic breast cancer in the bones is not curable, and the prognosis typically isn't as positive as earlier-stage cancers. Per the ACS, the five-year relative survival rate of a personal with distant breast cancer (aka metastatic cancer) is 28%. That means that people with metastatic breast cancer are about 28% as likely to be alive five years after diagnosis as people who don't have that cancer. 

However, data shows that metastatic breast cancer in the bones seems to have the best survival rate compared to other types of metastatic breast cancer. A 2019 study published in the journal BMC Cancer looked at five years of data to track the survival rates of stage four breast cancer patients and calculated specific rates based on the site of metastasis. The researchers found that patients with bone metastasis had the best overall survival (OS) rate of 50.5% after three years. This means that 50.5% of patients were still alive three years after diagnosis. (Meanwhile, people with brain metastases had a three-year OS rate of 19.9%. People with liver metastasis and lung metastasis had a three-year OS rate of 38.2% and 37.5% respectively.)

Other research has estimated that the one-year survival rate of metastatic breast cancer in the bones is 51% and the five-year survival rate is 13%

But these numbers are estimates, not foregone conclusions. A lot of factors can impact a person's prognosis, says Dr. Lin, including the type of cancer they have, where it spread, how old they are, and any other pre-existing conditions or health problems they have. And some cancers respond better to treatment than others, which absolutely affects a person's survival. 

All that said, treatment (and thus survival) for metastatic breast cancer overall has improved drastically over the past few decades. "We have many more options for [metastatic] cancer patients living longer," says Dr. Tsarwhas. The existing drugs and therapies can't cure, but they can greatly improve the length and quality of a patient's life—which really is the next best thing.

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