MBC stands for metastatic breast cancer, which is breast cancer that has spread beyond the breast to other parts of the body. It is classified as stage IV breast cancer and is the most advanced stage of the disease. About 6% of breast cancer cases are already metastatic at the time of first diagnosis, but most people with MBC were originally diagnosed with an earlier stage that later progressed. As of 2017, an estimated 150,000 women in the United States were living with MBC.
How Breast Cancer Becomes Metastatic
All breast cancer starts in breast tissue. It becomes metastatic when cancer cells break away from the original tumor and travel through the bloodstream or the lymphatic system to distant organs. Once these cells settle in a new location, they can form new tumors. Even though the cancer is now growing in the lungs or bones, it is still breast cancer, not lung cancer or bone cancer, because the cells themselves originated in the breast.
The most common sites where breast cancer spreads are the bones, liver, and lungs. It can also reach the brain and skin, though that happens less frequently. Cancer can spread to more than one site at the same time.
Symptoms by Location
The symptoms of MBC depend on where the cancer has spread. Bone metastases often cause deep, persistent pain in the back, hips, or ribs, and can weaken bones enough to cause fractures. Lung metastases may produce a chronic cough, shortness of breath, or chest pain. Liver metastases can lead to abdominal swelling, nausea, jaundice (yellowing of the skin or eyes), and fatigue. Brain metastases may cause headaches, vision changes, seizures, or difficulty with balance.
Some people with MBC have no obvious symptoms at first. The cancer is sometimes discovered during follow-up imaging after treatment for an earlier stage.
How MBC Is Diagnosed
If a doctor suspects breast cancer has spread, imaging tests are the first step. These typically include a chest X-ray, a bone scan, an ultrasound of the liver, and a CT scan. If any of these show something abnormal or unclear, more detailed imaging follows, such as MRI or a combined PET/CT scan, which can detect cancer activity throughout the body in a single session.
Whenever possible, doctors will take a tissue sample (biopsy) from the suspected metastatic site and examine it in a laboratory. This confirms that the cells are cancerous and determines the specific characteristics of the tumor, which directly shapes treatment decisions.
Subtypes That Shape Treatment
Not all breast cancers behave the same way. MBC is classified into subtypes based on which receptors the cancer cells carry on their surface. These receptors determine which treatments the cancer will respond to.
- Hormone receptor-positive (HR+): The most common subtype. These cancers are fueled by estrogen or progesterone and respond well to hormone-blocking therapies. Within this group, slower-growing tumors (called luminal A) respond strongly to hormone therapy alone, while faster-growing tumors (luminal B) often need chemotherapy added.
- HER2-positive: Making up 10 to 15% of breast cancers, these tumors overproduce a protein called HER2 that drives rapid cell growth. They are treated with drugs specifically designed to block this protein, combined with chemotherapy. These targeted drugs have dramatically improved outcomes for this subtype.
- Triple-negative (TNBC): These cancers lack all three common receptors (estrogen, progesterone, and HER2), which means hormone therapy and HER2-targeted drugs don’t work. Treatment has historically relied on chemotherapy, though newer targeted options are now available.
A biopsy of the metastatic tumor is important because the receptor profile can change as cancer progresses. A tumor that was HR-positive at the original diagnosis might test differently when it reappears in another organ.
Treatment Goals and Approach
MBC is generally not considered curable with current treatments. The goal shifts from eliminating the cancer entirely to controlling its growth, managing symptoms, and extending life. In oncology, this approach is sometimes described as treating cancer as a chronic disease. Cure requires no detectable disease for at least 10 years in breast cancer, which is extremely rare at stage IV.
That said, treatment can be highly effective at slowing the disease. Many people with MBC live for years while cycling through different treatment regimens. When one therapy stops working, doctors switch to another. The specific treatments available depend on the cancer’s subtype, where it has spread, and what the patient has already tried.
A newer class of drugs called antibody-drug conjugates has expanded options significantly, particularly for harder-to-treat subtypes. These drugs work by attaching a chemotherapy payload to an antibody that seeks out specific proteins on cancer cells, delivering the toxic drug directly to the tumor while sparing more healthy tissue. Three of these drugs are now FDA-approved for breast cancer, including options for HER2-low tumors (which were previously treated like triple-negative) and for triple-negative breast cancer specifically.
Survival Rates
The five-year relative survival rate for metastatic breast cancer is 33.8%, based on data from the National Cancer Institute’s SEER program covering 2016 to 2022. This means that about one in three people with distant-stage breast cancer are alive five years after diagnosis. For comparison, the five-year survival rate for breast cancer caught at its earliest, localized stage is nearly 100%.
These numbers reflect averages across all subtypes and ages. Individual outcomes vary widely depending on where the cancer has spread, how it responds to treatment, and which subtype is involved. HER2-positive MBC, for instance, has seen substantial survival improvements over the past two decades thanks to targeted therapies. Survival statistics also lag behind current treatments because they capture patients diagnosed years ago, before the newest drugs were available.
Living With MBC
Because MBC is managed long-term rather than treated and finished, it requires ongoing monitoring. This typically involves regular imaging scans, blood work, and check-ins with an oncology team. Treatment side effects vary depending on the drugs used but can include fatigue, nausea, bone thinning, and changes in appetite or mood.
Three out of four people living with MBC were originally diagnosed with an earlier stage. This means MBC can develop months or even years after someone was initially treated and believed to be cancer-free. The recurrence can happen because microscopic cancer cells survived the original treatment and remained dormant before eventually growing in a new location.