Bone metastases occur when cancer cells detach from their original tumor and travel to the bones, forming new growths. Also known as secondary bone cancer or metastatic bone disease, this condition differs from primary bone cancer, which originates directly in bone tissue. Bone metastases are more common than primary bone cancer, representing a spread from an existing cancer elsewhere in the body.
How Cancer Spreads to Bone
Cancer cells can spread to bones through a multi-step process. Cells break away from the primary tumor and invade surrounding tissues. These detached cells then enter the bloodstream or lymphatic system, circulating throughout the body. The rich arterial supply within bones makes them a common destination for these circulating tumor cells. Once in the bone marrow, these cells adhere to the bone matrix and begin to proliferate, disrupting normal bone remodeling.
While nearly any cancer can spread to bone, certain types are more prone to this process. Breast, prostate, lung, kidney, and thyroid cancers are among the most common primary cancers that metastasize to bone. The spine, pelvis, and long bones like the femur and humerus are frequently affected sites.
Recognizing the Signs and Symptoms
Bone pain is the most common and often the first symptom of bone metastases. This pain is typically described as a dull, persistent ache that can worsen at night or with rest. The pain may be localized to one area or spread throughout the body.
Fractures, known as pathologic fractures, can occur with minimal trauma due to bone weakening from metastatic lesions. These breaks often affect the ribs, vertebrae, and long bones of the legs. A sudden, sharp back pain might indicate a collapsing vertebra.
Spinal cord compression is a serious complication. It occurs when metastatic tumors in the spinal bones press on the spinal cord nerves. Symptoms include back pain, weakness or numbness in the legs and sometimes arms, and loss of bladder or bowel control.
High blood calcium levels, called hypercalcemia, can also develop when calcium is released from bones into the bloodstream due to bone breakdown. Symptoms of hypercalcemia include nausea, vomiting, constipation, extreme thirst, frequent urination, fatigue, and confusion.
The Diagnostic Process
Doctors employ various methods to confirm bone metastases, typically combining imaging and laboratory tests. Imaging tests are important for identifying changes within bone structure. X-rays can reveal osteolytic lesions, which appear as “holes” or dark spots in the bone, indicating bone destruction.
Bone scans, also known as bone scintigraphy, involve injecting a radioactive tracer that accumulates in areas of increased bone activity, appearing as darker spots. Positron emission tomography (PET) and computed tomography (CT) scans offer detailed cross-sectional images to identify metabolically active tumors and assess the extent of bone destruction. Magnetic resonance imaging (MRI) provides highly detailed images of bone marrow and surrounding soft tissues, useful for detecting early changes and assessing spinal cord compression.
Blood tests can provide supportive evidence, such as elevated calcium levels, which may suggest bone breakdown. While not specific for diagnosis, high levels of alkaline phosphatase or certain tumor markers can also suggest bone lesions. To definitively confirm the diagnosis, a biopsy is often performed, where a tissue sample is taken directly from the bone lesion for microscopic examination to identify cancer cells.
Goals and Types of Treatment
Treatment for bone metastases focuses on managing symptoms, preventing complications, and improving quality of life, as a complete cure is uncommon. Goals include slowing cancer growth, alleviating pain, stabilizing weakened bones, and preserving function. A multidisciplinary approach is often used to tailor treatment plans.
Systemic Treatments
Systemic treatments target cancer cells throughout the body. These include chemotherapy, which uses drugs to kill rapidly dividing cancer cells, and hormone therapy, which blocks hormones that fuel certain cancers like breast and prostate cancer. Targeted therapy uses drugs that attack cancer cells with specific genetic changes or proteins, while immunotherapy helps the body’s immune system recognize and destroy cancer cells.
Local Treatments
Local treatments are directed at specific bone tumors. Radiation therapy, delivered externally or internally, is a common approach to reduce pain and control tumor growth. External beam radiation therapy (EBRT) uses high-energy X-rays aimed at the tumor, often providing pain relief within weeks. Surgery may stabilize bones at risk of fracture, repair existing fractures, or relieve pressure on the spinal cord.
Bone-Modifying Agents
Bone-modifying agents strengthen bones and reduce the risk of skeletal complications. Bisphosphonates, such as zoledronic acid and pamidronate, inhibit osteoclasts (cells responsible for bone breakdown), reducing bone resorption and increasing bone density. Denosumab, a monoclonal antibody, targets and inhibits RANKL, a protein promoting osteoclast formation. These agents help reduce bone pain, lower fracture risk, and manage high blood calcium levels.
Managing Complications
Managing the complications of bone metastases aims to improve comfort and maintain function. Pain control often involves a combination of medications, from milder options like NSAIDs to stronger opioid medications. Nerve blocks, injecting anesthetic near nerves, can also provide localized pain relief.
Fracture Prevention and Management
Preventing fractures in weakened bones is a proactive measure. This may involve supportive braces or modifying daily activities to reduce stress on affected bones. If a bone is at high risk of breaking or has fractured, surgery may be necessary to insert metal rods, plates, or screws for stabilization. For spinal fractures, bone cement injection procedures like vertebroplasty or kyphoplasty can provide support and pain relief.
Spinal Cord Compression and Hypercalcemia
Spinal cord compression is a medical emergency requiring prompt action to prevent irreversible nerve damage. Treatment often involves immediate radiation therapy, sometimes combined with corticosteroids to reduce swelling. Emergency surgery may be performed to decompress the spinal cord by removing tumor tissue or stabilizing the spine. Severe hypercalcemia is managed by rehydration with intravenous fluids, often accompanied by medications like bisphosphonates or denosumab to lower blood calcium levels by inhibiting bone breakdown.