What Are Bone Metastases

Overview

The process of cancer cells breaking off from their original site, travelling to another part of the body and forming new tumours in a secondary site is called metastasis. The cancer that forms in the new site can also be called a metastasis (plural: metastases), a secondary cancer or a metastatic cancer. Secondary cancers that form in the bone are called bone metastases (meh-TAS-tuh-seez), sometimes abbreviated to bone mets.

Causes of bone metastases

The original site where a cancer begins to grow is called a primary cancer. Cells from the primary cancer can break off and travel to a different organ or tissue of the body, usually through the blood or lymph systems. The most common sites for metastases are the liver and the lung, followed by the bones.¹ It is not fully understood why some cancers spread to different parts of the body and others do not; nor why some spread to one organ and not another.

Most types of cancer have the potential to spread to the bone as bone metastases but some are more likely to do so. These include:

• Prostate cancer
• Breast cancer
• Lung cancer
• Thyroid cancer
• Kidney cancer
• Myeloma (a form of blood cancer)

Primary breast cancer and prostate cancer are particularly likely to spread to bones.² 

The bone metastases are still made of cells from the primary cancer. A metastatic bone tumour which originated from primary breast cancer, for example, would be composed of breast cancer cells, not of bone cells. The cancer is classified by its site of origin, so in this example, the secondary cancer would be called metastatic breast cancer in the bone or breast cancer bone metastases.³ 

Metastatic cancers in the bone are different from primary bone cancers. In primary bone cancer, the cancer originates in the bone. In bone metastasis, the cancer originates elsewhere and spreads to the bone. Bone metastases are more common than primary bone cancers, particularly in adults.²

Primary cancers can spread to many bones of the body, but the most common ones are:

• Long bones of the arm and leg
• Spine
• Pelvis
• Ribs
• Skull

Bone damage due to metastasis

Healthy bone is continuously remodelled throughout life, with old cells being broken down and replaced by new ones.^4,5 This balance of bone resorption and regeneration is known as bone remodelling or bone metabolism. This process helps to preserve the structural integrity of the skeletal system, repair areas of microdamage and maintain the correct balance of calcium and phosphorus minerals in the body.⁶ 

These metabolic pathways, however, are complex and finely balanced⁴ and can be significantly disrupted by metastatic bone disease.⁷ As a result of this disruption, there can be either an increase in the rate of cell breakdown (resulting in bone loss) or an increase in the generation and deposition of new bone tissue (resulting in abnormal bone growth).

An increase in bone cell destruction and resorption can lead to the creation of holes called osteolytic lesions. These weaken the structure of the affected bone, making it more fragile. 

An increase in new cell generation can lead to areas of abnormal new bone growth called osteoblastic or sclerotic lesions.² These new growths can cause deformity, as well as weaken the bones and make them susceptible to collapse. 

Bone metastasis is classified in each case as either osteolytic, osteoblastic, or mixed, depending on the main mechanism of disruption to the remodelling process.² 

Signs and symptoms of bone metastases

The following are the most common signs and symptoms of bone metastasis:

• Pain: Bone pain is the most common symptom of bone metastases⁸ and often the first symptom to be experienced.⁹ Pain is often poorly localised² and may be described as ‘gnawing’. Bone pain may initially come and go and be eased by movement, but later may become constant and more severe. This type of pain is often worse at night and not relieved by rest
• Fractures: Bones weakened by metastases can become fragile and break more easily (pathological fractures). The bones of the arms, legs and spine are the most common sites. Sudden, severe pain and inability to move may indicate a fracture. Breaks in the femur (the long bone of the thigh) account for over 50% of all cases
• Loss of sensation in arms/legs, weakness in arms/legs, loss of bladder/bowel control: These symptoms can be caused by compression of the spinal cord due to bone metastases in the spine
• Tiredness, weakness, shortness of breath: Caused by anaemia resulting from changes to red blood cells
• Loss of appetite, increased thirst and urination, constipation, nausea/vomiting, tiredness, confusion: Caused by hypercalcaemia (increased calcium levels in the blood) due to excessive breakdown and resorption of bone cells
• Recurrent infections: Due to lowered immunity resulting from changes to white blood cells
• Easy bruising/bleeding: Due to blood platelets being affected

The collective term skeletal-related events (SRE) is used to encompass complications arising in the skeleton due to bone metastasis.¹⁰

Bone metastases can develop several years after the primary cancer. In most cases, people who develop these symptoms have a known history of primary cancer. However, in a certain number of cases (estimated 20% of cases), bone metastases will be the first sign of cancer. 

Management and treatment for bone metastases

Bone metastasis can not usually be cured but treatment may be able to shrink the secondary cancer or slow its growth, as well as to treat symptoms to optimise quality of life.² 

Treatment is usually multidisciplinary,¹¹ involving several different components to target both the disease process and symptoms, and is highly tailored to the individual case.⁸ Factors which determine the choice of a treatment plan include the primary cancer type, how localised or widespread the metastases are, prior history of treatment and response, symptoms and general health.²

Treatment options (categorised by aims) include:

• Bone pain relief through:
  • Radiotherapy (also called radiation therapy)^2,8 – use of high energy X-rays, delivering ionising radiation to kill cancer cells and shrink tumours, delivered externally or internally
  • Pain medication¹²
  • Steroid medication to reduce swelling and inflammation

• Targeting of primary cancer through:
  • Ablation – insertion of a needle or probe into a tumour and application of heat, cold or a chemical to destroy it²
  • Stereotactic radiosurgery and stereotactic body radiotherapy (use of targeted radiation beams to target tumour cells)²
  • Chemotherapy – anti-cancer drug treatment used to kill cancer cells 
  • Targeted cancer therapy – a more specific form of drug treatment to target cancer cells which may involve less damage to healthy cells
  • Radiotherapy
  • Hormone therapy – to treat hormone-sensitive cancers
  • Surgery to remove the tumour or remove hormone-producing organs in hormone-sensitive cancers

• Prevention of pathological fractures and other skeletal complications:
  • Surgery⁸ such as stabilisation with plates and screws or injection of bone cement
  • Bone-strengthening medications such as bisphosphonates and denosumab^2,8

• Treatment of pathological fractures and other skeletal complications once they have occurred through surgery²

In addition, treatment for symptoms such as tiredness may be given, as well as physiotherapy to help with strength and mobility.

Research in this area is ongoing and best practice in cancer treatment and metastatic disease continues to evolve. Updates on research in this area are available from websites such as The National Cancer Institute (US), the American Cancer Society and Cancer Research UK.

Diagnosis of bone metastases

The following tools are commonly used to diagnose bone metastasis:⁹

• X-ray: to provide images of bones and joints
• CT (Computed Tomography) scan: to provide more detailed images of bones and soft tissues
• MRI (Magnetic Resonance Imaging) scan: to provide more detailed images of bones and soft tissues
• Bone scan: to highlight areas of abnormal change in bones 
• PET (Positron Emission Tomography) scan: to highlight areas of abnormal cell activity
• Blood/urine tests: to identify abnormal levels of different substances present in blood or urine 
• Biopsy: for more detailed examination of a tissue sample

FAQs

How can I prevent bone metastases?

Bone metastases are cancers in the bone which have spread from primary cancers elsewhere in the body. The risk of developing these secondary bone cancers is therefore reduced with early, rapid and effective treatment of the primary cancer. This may be complemented with adjuvant therapy, used to reduce the risk of the cancer returning.  

Patients who have had primary cancer and are deemed to be at risk of bone metastasis may be treated preventatively with bone-strengthening medications such as bisphosphonates and denosumab.^2,8

How common are bone metastases?

The overall incidence of bone metastasis is difficult to quantify as there is significant variation in its occurrence depending on factors such as the type of primary cancer, the stage of cancer at diagnosis and the length of time after a primary cancer diagnosis. In a study carried out in 2022 by Hernandez et al.,¹³ 6.9% of cancer patients overall developed bone metastases, with an average time of occurrence of 400 days following the diagnosis of primary cancer. Of this group, the highest incidence was seen amongst prostate cancer patients, with 18 - 29% of patients developing bone metastases within 5 years after diagnosis. Patients whose primary cancer was diagnosed at an advanced stage (stage IV) showed a high likelihood of developing bone metastasis, with 11% incidence within 30 days post-diagnosis. 

Who is at risk of bone metastases?

Most types of cancer have the potential to spread to the bone to form bone metastases, but some are more likely to do so than others. Patients with a history of primary breast or prostate cancer are at greatest risk of the development of secondary bone cancers.² The risk also increases with primary cancers that are already at an advanced stage when diagnosed.¹³

The process of metastasis, including when and why it occurs, is not fully understood. Research into this area is ongoing. 

When should I see a doctor?

The most common symptom of bone metastasis is bone pain. This may be poorly localised and worse at night or when resting. Bone metastases increase the risk of bone fracture and at times, this may be the first sign. A sudden, severe pain and inability to move may indicate that you have a broken bone. 

Other symptoms include a reduction in bladder and bowel control, numbness or weakness in arms and legs, general feelings of tiredness, increased thirst, nausea, vomiting, confusion, and constipation. 

If you experience any of these symptoms, consult a healthcare professional. In particular, seek immediate help if you experience symptoms like sudden loss of bladder and bowel control and significant numbness/weakness in your arms and legs. These may indicate spinal cord compression which is considered a medical emergency and requires urgent evaluation.

Summary

Bone metastases are secondary cancers that have spread to the bone from elsewhere in the body. These metastatic cancers are made of cells from the primary cancer that have travelled to the bone, usually via the blood or lymph system. Bone metastases cause pain and weaken the structure of the skeletal system, increasing the risk of fractures and other skeletal complications. Metastatic bone cancer is usually incurable but treatments may be able to slow or stop its growth. Treatments may also be able to strengthen bones to prevent fractures and to help reduce symptoms such as bone pain.

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