When breast cancer spreads, or metastasizes, it is referred to as stage IV or advanced breast cancer. This means cancer cells from the primary tumor in the breast have traveled to other parts of the body. The bones are a common site for breast cancer to spread, and for more than half of women with metastatic breast cancer, the bones are the first site of metastasis.
It is important to understand that breast cancer in the bone is not bone cancer. The cancerous cells are identical to the original breast cancer cells, so it is treated with therapies targeting breast cancer. While this condition signifies an advanced stage, numerous treatments can manage symptoms and slow its progression.
The Process of Metastasis to Bone
The journey of a cancer cell from the breast to the bone begins when cells detach from the primary tumor and enter the body’s circulatory or lymphatic systems. These traveling cells are carried throughout the body, but their ability to establish a new tumor depends on the environment they encounter. This is often explained by the “seed and soil” hypothesis, where cancer cells are the “seeds” and a receptive organ like bone is the “soil.”
The bone marrow provides a fertile environment, rich in growth factors that help breast cancer cells thrive. This supportive microenvironment is why bone is a frequent destination for metastasis. Once breast cancer cells settle in the bone, they disrupt the normal process of bone remodeling—the continuous cycle of old bone being broken down and new bone being built.
This disruption leads to two primary types of bone lesions. Osteolytic lesions occur when cancer cells accelerate bone breakdown, creating weak spots or holes. In contrast, osteoblastic lesions result from the over-activation of bone-forming cells, causing areas of dense but structurally unsound bone. Many people with bone metastases will have a combination of both types.
Symptoms and Diagnosis
The most common initial sign of breast cancer in the bones is a new, persistent pain in a specific area. This pain may start as a dull ache and become more severe over time, often worsening at night. Because the bone is weakened by the cancer, it can lead to a pathologic fracture, which is a break that occurs from a minor injury or during normal activities.
If metastases develop in the spine, they can cause spinal cord compression as the tumor presses on the spinal cord. This is a medical emergency requiring immediate attention to prevent permanent damage. Symptoms include weakness or numbness in the limbs, difficulty walking, and loss of bladder or bowel control.
Another serious complication is hypercalcemia, an elevated level of calcium in the blood. This happens when bone breaks down rapidly, releasing too much calcium into the bloodstream, and also requires urgent care. Symptoms can include nausea, vomiting, constipation, excessive thirst, and confusion.
To determine if breast cancer has spread to the bones, doctors use several imaging tests. A bone scan is often performed first to detect abnormal bone activity throughout the body. Other imaging techniques can provide more detailed pictures to confirm the presence of tumors and assess their size and location, including:
- X-rays
- CT scans
- PET scans
- MRIs
While imaging tests can strongly suggest bone metastases, a definitive diagnosis often requires a bone biopsy. During this procedure, a small sample of the suspicious bone tissue is removed with a needle and examined under a microscope. Confirming that the cells are breast cancer cells verifies the diagnosis.
Systemic and Localized Treatment
Treating breast cancer that has spread to the bone involves combining systemic therapies, which treat cancer throughout the body, and localized treatments, which target specific bone tumors.
Systemic Treatments
Systemic therapies circulate through the bloodstream to reach cancer cells wherever they may be. The specific type of treatment is chosen based on the characteristics of the breast cancer cells, such as their hormone receptor (ER/PR) and HER2 status. For cancers that are ER-positive or PR-positive, hormone therapy is a primary treatment. These drugs work by blocking the effects of estrogen or lowering its levels in the body, which starves the cancer cells of the hormones they need to grow.
For cancers that are HER2-positive, targeted therapies are used to specifically attack the HER2 protein on the surface of the cancer cells, controlling their growth. Chemotherapy may be used for cancers that are hormone receptor-negative and HER2-negative (triple-negative breast cancer) or if the cancer is growing quickly. Immunotherapy, which helps the body’s own immune system attack cancer cells, has also become an option for some individuals.
Localized Treatments
Localized treatments are aimed directly at tumors within the bone to relieve pain and prevent complications, known as skeletal-related events (SREs). Bone-modifying drugs, such as bisphosphonates and denosumab, are a standard part of care. These medications work by slowing down the cells that break down bone, which helps to strengthen the bones, reduce the risk of fractures, and lower high blood calcium levels.
Radiation therapy is another common localized treatment that uses high-energy rays to damage or destroy cancer cells in a specific area. This is effective for reducing pain from bone tumors and can also help shrink tumors pressing on nerves or the spinal cord. In some situations, surgery may be necessary to stabilize a bone that has fractured or is at high risk of fracturing, or to relieve pressure on the spinal cord. This often involves inserting metal rods or plates to support the weakened bone.
Pain Management and Safety
Pain management is an important aspect of daily life. The approach to controlling pain is often multi-faceted, starting with over-the-counter pain relievers and progressing to stronger prescription medications as needed. Palliative care specialists can be an invaluable resource, offering expert guidance on complex pain management and addressing other symptoms to improve overall comfort and quality of life.
Preventing fractures requires a proactive approach. This includes being mindful of movements, avoiding heavy lifting, and taking precautions to prevent falls. Physical and occupational therapists can help develop safe exercise routines and strategies for performing daily activities that protect weakened bones.
Prognosis and Living with Bone Metastases
While stage IV breast cancer is not considered curable, it is a treatable condition. The primary goals of treatment shift from cure to control: managing the cancer as a chronic disease, slowing its progression, relieving symptoms, and ensuring the best possible quality of life.
Many people live for several years with bone metastases, and advancements in treatment continue to improve outcomes. A person’s prognosis can vary significantly and is influenced by several factors. These include the specific subtype of the breast cancer (ER, PR, and HER2 status), the extent of the metastases, and the individual’s overall health and response to treatment.
Survival statistics, such as median survival rates, are often discussed but can be misleading. These numbers are averages calculated from large groups of people and cannot predict what will happen for any single individual. Open and honest communication with the healthcare team is important for understanding the specific situation and developing a personalized care plan that aligns with individual goals and priorities.