What is Bone Metastasis?
Metastatic bone cancer, also called secondary bone cancer, happens when tumors from other parts of the body spread to the bone. The bone has a rich supply of blood, making it a frequent place for these types of tumors to go. In fact, after the lungs and liver, the skeleton is the third most common place for the disease to spread. Primary bone cancer, on the other hand, is fairly rare and starts in the bone itself due to uncontrolled growth of cancerous cells. Remember, the term primary bone cancer means that the cancer started in the bone and wasn’t spread from another area. Cases of secondary bone cancer can vary widely, and may respond to different treatments like chemotherapy, radiation, or surgery.
What Causes Bone Metastasis?
Carcinoma, a type of cancer arising from the cells that make up the skin or the tissues lining the organs, is the most frequent cause of secondary bone cancer – which means it’s a cancer that started elsewhere in the body and later spread to the bones. Many cases of secondary bone cancer come from spread out breast, lung, and prostate cancer. However, kidney and thyroid cancer can also spread to the bone.
Carcinomas develop from a type of tissue called epithelial tissue, which forms the outer layer of our body and internal organs. There are also types of cancers that spread to the bone but they are not from the epithelial cell lines.
Some blood cancers, like multiple myeloma and lymphoma, can affect the bone too. Furthermore, sarcomas, cancer that originates from mesenchymal tissues (which include bone, cartilage, fat, muscle, and other connective tissues), can also spread to the bone later in their progression.
Risk Factors and Frequency for Bone Metastasis
Bone metastases, which are cancer cells that have spread to the bones, affect the length of survival for different types of cancers. For instance, people with lung cancer who develop bone metastases typically live 6 to 7 months, while those with breast or prostate cancer can live for several more years. Bone metastases can come in various forms, including a single bone lesion, a few metastases (known as oligometastatic disease), multiple bone metastases, or a combination of bone and organ metastases.
- In a study, researchers found that the likelihood of developing bone metastases in the United States increased over time – with rates of 2.9% at 30 days, 4.8% at one year, 5.6% at two years, 6.9% at five years, and 8.4% at ten years.
- Among all cancers, prostate cancer patients were most likely to develop bone metastases (18% to 29%) followed by patients with lung, kidney, or breast cancer.
Signs and Symptoms of Bone Metastasis
Patients with cancer that has spread to their bones often have increasing pain, or experience other bone-related problems. Sometimes, a low-energy injury that doesn’t heal as expected can be a sign of these issues. Often, pain medications don’t fully relieve the symptoms. Doctors will look out for warning signs such as pain that disrupts sleep, unexplained weight loss, pain when carrying weight, or a growing lump in the concerning area. They’ll also examine the skin to look for swelling, soreness, wounds, or changes in appearance. A basic support like a splint can be used to protect the affected limb and prevent further injury. The most commonly affected bones are the vertebrae, followed by the femur, pelvis, ribs, sternum, upper arm bone, and skull.
There are certain complications that doctors look for when a patient with cancer is suspected to have cancer spread to the bones:
- Bone pain: This is commonly experienced by patients, and often comes on gradually. The pain is usually described as dull and boring, and worst at night.
- Nerve root or spinal cord compression: If the cancer spreads to the nerves, it can cause a sharp, radiating pain.
- Spinal cord compression: This is a serious condition that occurs when cancer in the vertebrae puts pressure on the spinal cord. Symptoms can include back pain, weakness in the limbs, tingling and numbness, and in later stages, problems with bowel and bladder control or sexual dysfunction. Prompt treatment can prevent permanent damage.
- Hypercalcemia: This is a high level of calcium in the blood, and in cancer patients, it often indicates a poor prognosis. It is associated with nausea, loss of appetite, stomach pain, constipation, and changes in mood or mental state. Hospitalization and intravenous fluids are usually needed to treat hypercalcemia.
- Pathological fractures: These occur when the cancer has weakened a bone to the point that it breaks under normal pressure. Symptoms can vary depending on the location of the break, but common signs include persistent pain and pain that worsens while sitting or standing. Bone fractures can lead to further complications like nerve pain, physical deformities, and lack of mobility.
- Myelophthisis: This is a condition where the cancer spreads to and damages the bone marrow, leading to symptomatic anemia. In late stages, it can lead to a decrease in all types of blood cells.
Testing for Bone Metastasis
Detecting bone metastases, or cancer that has spread to the bones, is important not only for understanding the stage of the disease but also for planning treatment strategies. These strategies can help decrease the impact of the disease and increase survival rates.
Metastases in the bones can appear in three different ways on x-rays or scans: osteolytic (where the bone is being destroyed), sclerotic (where new bone is forming), or a mix of both.
Simple x-rays are often the first images taken in patients who complain of bone pain. They are useful for spotting abnormal growths or bone fractures that can result from cancer weakening the bones. But X-rays are better at catching osteolytic lesions – or areas where bone is being destroyed by cancer – once they are quite large and have caused significant bone loss. Sclerotic lesions, where new bone is being formed, look different–they appear abnormally thick and well-defined. However, the downside is that x-rays are not very sensitive, and could miss smaller metastases or ones that do not cause symptoms.
Computed tomography (CT) scans, however, are more sensitive than x-rays and can better visualize both the outer and inner structure of bones. CT scans are also useful for measuring the size of the cancer and assessing how well treatment is working. They are particularly good at looking at the ribs because of their high detail. The scans can also help doctors in planning surgeries for metastatic disease, particularly around the shoulder or pelvis.
Magnetic Resonance Imaging (MRI) is even more sensitive than CT scans for spotting bone metastases, correctly identifying them 95% of the time. MRIs are especially useful because they can see changes in bone marrow before bone lesions become visible to x-rays. MRIs can also be used safely in pregnant women and can detect spinal cord compression, a serious complication that can occur if cancer spreads to the spine. On an MRI, bone metastases show up as low in one type of MRI sequence (T1 weighted) and high intensity on another type (T2).
Nuclear medicine scans are another type of imaging used to detect bone metastases. They involve injecting a small amount of a radioactive material, which then collects in areas of the bone affected by cancer. There are several variations of these scans, and while some are very sensitive and can provide an image of the whole skeleton, they can sometimes have difficulty distinguishing between harmless and cancerous bone lesions.
Combining different imaging techniques can improve both the image quality and the diagnostic accuracy. For instance, PET/CT scans, which merge a nuclear medicine scan (PET) with a CT scan, are 100% accurate in detecting bone metastases.
In addition to imaging, blood tests can provide further information. Routine tests such as checking the number and types of blood cells can reveal abnormalities caused by advanced disease while other tests can show in real-time how aggressive the cancer is in the bones. Classic markers include high levels of calcium and an enzyme called alkaline phosphatase which are released when the bones are actively being destroyed by cancer.
Finally, established scoring systems, like the Mirel classification and spinal stability tests, help doctors decide whether surgery is needed to prevent fractures from occurring in weakened bones.
Treatment Options for Bone Metastasis
A multidisciplinary approach that considers various factors is essential when treating bone metastases (cancer spread to the bone). The main goal is to maintain the patient’s quality of life by managing painful symptoms, preventing skeletal-related events (SREs), and controlling the cancer whenever possible.
Effective pain management is needed for the severe pain that often accompanies bone metastasis. Usually, nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, are used first and can be increased or used in combination with narcotics as necessary. Glucocorticoid medications can provide additional pain relief but should be discussed with the entire healthcare team. This is important because glucocorticoids can affect the accuracy of a biopsy, which is a cancer-testing procedure.
To decrease the complications associated with bone metastases, osteoclast inhibitors, like bisphosphonates and denosumab, can be used. Osteoclasts are cells that break down bone. By limiting these cells, the osteoclast inhibitors help prevent bone damage and can also assist in reducing bone pain.
Local radiotherapy, which targets the cancer cells in the bone, is primarily used for pain relief in patients with bone metastasis and to ensure continued bone healing after surgery. The standard treatment involves the use of external beam radiotherapy, which can reduce pain. In some cases, a form of radiotherapy called stereotactic body radiotherapy (SBRT) may be used. This technique precisely targets the cancer cells and spares healthy tissues.
Bone-targeted radiopharmaceutical therapy uses radioactive materials to treat widespread bone pain related to bone metastases. This type of treatment is typically used for patients with prostate and breast cancer.
Chemotherapy, treatment that uses medication to kill cancer cells, can also be an effective strategy for pain relief. By reducing the size and spread of the tumor, this form of systemic therapy can help alleviate symptoms.
When there’s a risk or occurrence of fractures due to cancer spread in the bones, surgery can be done to either prevent the fracture or stabilize the bone. This helps maintain the patient’s mobility. Similarly, surgery can provide protection and stabilization to the spinal cord if there’s a threat of compression. In certain cases where the cancer is limited in its spread, a surgery to remove the cancerous tissue may be considered.
For patients who continue to experience pain after radiotherapy or whose pain reoccurs, local removal of the cancer cells through techniques like radiofrequency ablation, cryoablation, or focused ultrasound can be considered.
What else can Bone Metastasis be?
When a doctor suspects that cancer has spread to the bones, they will consider several potential diagnoses based on the specific bone affected. This could include:
- A primary bone sarcoma, which is a type of bone cancer
- Multiple myeloma, a cancer of the blood cells
- Primary malignant lymphoma of the bone, a cancer of the lymphocytes
- Secondary or post-radiation sarcoma, a rare type of cancer that can develop after radiation therapy
- Osteomyelitis, an infection in the bone
To determine the correct diagnosis, doctors must distinguish between acute osteoporotic fractures and fractures caused by metastatic cancer through X-rays or other imaging techniques. In osteoporosis, the outer layer of bone (known as the cortical bone) might be intact, while in cases of metastatic cancer this area of the bone is usually damaged.
Possible Complications When Diagnosed with Bone Metastasis
Surgery for metastatic bone cancer is a complex process and can cause several complications. Ensuring the correct diagnosis before starting treatment is crucial as it can lower the risk for the patient. The complications from surgery can entail:
- Pain
- Bleeding
- Infection
- Damage to nerves and vessels during surgery, leading to numbness or paralysis
- Cancer returning
- Cancer spreading
- Death from surgery complications or the spread of cancer
