The vertebra is the most common site affected, followed by the femur, pelvis, ribs, sternum, proximal humerus, and skull. Bone metastases may be asymptomatic or manifest in a variety of ways termed skeletal-related events (SRE) as a result of the destruction of normal bone architecture. The following further describes SRE: 

1. Bone pain: Pain associated with bone metastasis is a frequent symptom. It is typically gradual in onset and described as a dull, boring pain which is worse at night.
2. Nerve root or spinal cord compression: Bone metastases causing nerve root compression can present as radicular pain different from mechanical pain.
3. Spinal cord compression: As the vertebra is the most common site of metastasis, a significant complication includes spinal cord compression, which is an oncologic emergency. Metastatic spinal cord compression occurs either through pathological vertebral collapse or direct epidural extension. Patients may present initially with back pain. Limb weakness is the second most common symptom of cord compression. Sensory symptoms include paresthesias and numbness at and below the level of cord compression. Autonomic dysfunction, including bowel and bladder incontinence and impotence, is typically a late presentation. Early recognition, workup, and prompt surgical consultation are pertinent to prevent permanent neurological damage with resultant paraplegia. 
4. Hypercalcemia: In the setting of malignancy, this can be multifactorial and confers a poor prognosis overall. Osteolytic bone metastases are associated with 20% of the cases of hypercalcemia of malignancy. In osteolytic metastases, enhanced osteoclastic bone resorption occurs as a result of the release of humoral factors by tumor cells that in turn stimulate osteoclasts and lead to unchecked bone resorption and hypercalcemia. Symptoms of hypercalcemia include nausea, anorexia, abdominal pain, constipation, and mental status changes. Immediate treatment for hypercalcemia includes IV hydration 
5. Pathological fractures: Bone metastases cause bone destruction, leading to complete or impending fracture at the site of pathology either spontaneously or with minimal trauma. The presentation is dependent on the site of the fracture; however, constant pain is a prevalent symptom. Fractures of the thoracic and lumbar spine present with pain characteristically worse with sitting or standing. Pathologic fractures result in significant morbidities resulting from pain, radiculopathy (e.g., sciatica with pelvic fracture), deformities, and immobility. 
6. Myelophthisis: Symptomatic anemia resulting from infiltration of the bone marrow with metastatic tumor cells. Pancytopenia may also be present in late stages. 

It is pertinent to identify bone metastasis early, both for staging and prognostication as well as the implementation of prophylactic and treatment strategies which may lead to decreased morbidity and mortality. 

Bone metastases can be characterized as osteolytic, sclerotic, or mixed on imaging studies. 

Plain Radiograph

X-rays or plain radiograph is the initial imaging of choice in patients presenting with bone pain. Plain films are used to assess abnormal radionuclide uptake or to detect pathological fractures. 

Plain radiography best detects osteolytic lesions, but they may not be apparent until they are greater than 1 to 2 centimeters and with loss of 50% of the bone mineral content at the site of disease. Osteolytic lesions are seen as thinning of trabeculae and ill-defined margins on radiographs, while sclerotic lesions appear nodular and well circumscribed as a result of thickened trabeculae. Plain films tend to be insensitive, especially in detecting bone metastases and with asymptomatic and subtle lesions. Progression of disease and response to therapy can be monitored with plain films and further correlation with other modalities. Sclerosis or new bone formation in osteolytic metastatic lesions is demonstrated by the sclerotic rim of reactive bone, which starts at the periphery and eventually involves the center with continued healing. Purely sclerotic lesions are more difficult to assess. Major disadvantages of plain radiographs include poor sensitivity.

Computed Tomography

Computed tomography (CT) is more sensitive (74%) than plain radiographs. It is useful in the evaluation of cortical and trabecular bone as well as in the assessment of osteolytic and sclerotic lesion. CT scan is advantageous as it can determine staging and treatment response of other organs in addition to bone and objectively assess reactive sclerosis by calculating the change in Hounsfield units. Ribs are better evaluated with CT due to the high cortex to marrow ratio. 

MRI

In the detection of bone metastasis, MRI demonstrates a sensitivity of 95% and specificity 90%. MRI is also more advantageous than a bone scan as it can detect marrow involvement before the development of osteoblastic lesions. It can be used with women who are pregnant and used to detect spinal cord compression. Bone metastases manifest as low T1 signal and high intensity on the T2 weighted sequence. Whole-body MRI requires 40 to 45 minutes to perform and involves short-tau inversion recovery (STIR) and/or T1-weighted sequences.

Nuclear Medicine 

Nuclear medicine scans also are used to detect bone metastases using osteotropic radioisotopes; these include skeletal scintigraphy, SPECT, and PET scan. 

Skeletal scintigraphy or bone scan is the most commonly used radionuclide imaging which uses 99mTc-MDP employed in the detection of skeletal metastases. Radioisotopic imaging methods depict bone metastatic lesions as areas of increased tracer uptake.

Bone scan provides the advantage of scanning the whole skeleton and has a high sensitivity (78%) therefore resulting in early diagnosis. When osteoblastic activity is prominent, the lesions are readily detected using radionuclide bone scanning. However, bone scans have a low specificity for differentiating between benign and malignant bone lesions and for the detection of predominantly osteolytic lesions. Bone scans can be used to monitor the progression of disease and response to treatment.

SPECT uses 99mTc-MDP radioisotopes uptake to detect bone lesions; however, images are acquired in cross-sectional rather than a planar fashion. SPECT has a higher specificity of 91% compared to skeletal scintigraphy.

PET is a nuclear medicine technique that uses the radiotracers 18F FDG or 18FNaF for the detection of skeletal metastases. 18F FDG PET scan identifies bone metastases based on a high glucose metabolism exhibited by neoplastic cells. PET has a better spatial resolution compared to skeletal scintigraphy. 18F NaF-PET is proven to be substantially more sensitive and specific than bone scan and SPECT for the detection of bone metastases.

Combining imaging techniques and modalities allows for improved visualization both anatomically and functionally, leading to increased diagnostic accuracy. One example of this is the 18F-Sodium fluoride (18F-NaF) PET/CT bone scanning which has a significantly greater sensitivity (100%) and specificity (97%). Other hybrid imaging techniques include SPECT/CT, PET/CT, and PET/MRI.

Blood tests can aid in supporting the diagnosis of bone metastases. Complete blood count and a comprehensive metabolic panel should be obtained routinely. CBC may reveal anemia, thrombocytopenia, or pancytopenia in late stages. Serum calcium and alkaline phosphatase may be elevated due to ongoing osteolysis. Bone turnover markers are still being studied as indicators of bone resorption. Tartrate-resistant acid phosphatase has been proven to elevated in patients with breast and prostate cancer with bone metastases.

Objective scoring models such as the Mirel classification system for long bones and assessment of spinal stability in addition to imaging criteria are used to determine the surgical necessity for impending pathological fractures.

The therapeutic approach to bone metastases should be a multidisciplinary approach targeted at preserving the quality of life, including pain control, minimizing SREs, and achieving local tumor control. It is pertinent to consider a multitude of factors including the extent of disease spread, performance status, impending fracture, and side effects when creating the initial approach for the treatment of bone metastases. [8][9][10][11]

A major aspect of treating bone metastases is analgesia/pain control for the debilitating pain that occurs with bone metastases. Pain control can be initiated with NSAIDs and titrated up to or in conjunction with narcotics as needed for symptom relief. Glucocorticoids may also be useful for additional pain control. 

Osteoclast inhibitors (bisphosphonates and denosumab) decrease morbidity and mortality associated with bone metastases as they reduce skeletal-related events and can be used for analgesia to some extent. 

Local radiation for symptomatic bone metastases is a significant component of the palliative approach in providing analgesia. It is also used postoperatively to consolidate continued bone healing. External beam radiation is the standard approach for painful bone metastases and is beneficial in reducing pain by up to 50% to 80%. Several studies have proven that a single 8 Gy fraction compared to more prolonged or fractionated radiation is non-inferior; however, it may carry a higher need for pretreatment (20 % vs. 8%). Stereotactic body radiation therapy spares the normal tissue while delivering highly conformal radiation to the affected area. There are no clear guidelines on stereotactic body radiotherapy (SBRT) use however it may be indicated over external beam radiation therapy (EBRT) in specific instances of bone metastases specifically vertebral from certain radio-resistant neoplasms. 

Bone-targeted radiopharmaceutical therapy (e.g., beta-emitting agents strontium-89, alpha-emitting radium-223) provides the specific advantage of treatment of diffuse pain associated with osteoblastic bone metastases. It is typically used in bowel movement associated with prostate and breast cancer or for analgesia in radiation therapy for refractory pain. 

Systemic chemotherapy when amenable, aimed at the primary tumor can also provide analgesia by reduction of tumor size and control of tumor spread.

Surgery is indicated for impending or complete fracture, mechanical stability, and spinal cord compression. In cases where the spread of the primary cancer is limited to a single bone lesion, en bloc resection of the metastasis can be done by means of local tumor control. 

Local ablation via radiofrequency ablation (RFA), cryoablation, and focused ultrasound (FUS) should be considered for patients with persistent pain following radiation therapy or patients with recurrent pain. 

The differential diagnosis for bone metastases includes chondrosarcoma, primary malignant lymphoma of the bone, multiple myeloma, post-radiation sarcoma, and osteomyelitis. 

A distinction between acute osteoporotic fractures versus metastatic fractures should be made on radiographic imaging. In osteoporosis, the cortical bone may be preserved; however, cortical bone destruction is typical with metastatic cancer.

The management of a patient with bone metastases is an interprofessional. The therapeutic approach to bone metastases should be an interprofessional approach targeted at preserving the quality of life, including pain control, minimizing SREs, and achieving local tumor control. It is pertinent to consider a multitude of factors including the extent of disease spread, performance status, impending fracture, and side effects when creating the initial approach for the treatment of bone metastases. 

A major aspect of treating bone metastases is analgesia/pain control for the debilitating pain that occurs with bone metastases. Pain control can be initiated with NSAIDs and titrated up to or in conjunction with narcotics as needed for symptom relief. Glucocorticoids may also be useful for additional pain control. 

It is important to understand that many of these patients are frail and terminal and unnecessary investigations and procedures should be avoided. Comfort care or hospice care is ideal for many of these patients. A pain specialist, social worker and a hospice nurse must be involved in the care of these patients- the aim is to ensure that the patient has a decent quality of life. The majority of patients with bone metastases are dead within 4-12 months. [12](Level V)