Metastasis refers to the spread of cancer cells from the part of origin (primary site) to other parts (secondary sites) in one's body. Cancer cells are genetically modified cells that multiply and grow rapidly. The uncontrolled propagation, abnormal microscopic structure, and poor differentiation of cancer cells give them the malignant (rapid and worse advancement) tendency. This tendency empowers the cancer cells to invade the blood vessels and circulate all over the body through blood. Eventually, now these cells can enter and affect any system of the body. Metastasis is one of the key characteristic signs of cancer, differentiating them from non-cancerous growths.
Bone metastasis means the invasion of cancer cells into the bone from the primary site. The skeleton remains the third most common site of metastatic cancer after the lungs and liver. Most of the bone metastasis dissipates from the breast, lungs, and prostate cancers. Bone metastasis may confer multiple bony lesions (most common), single bone involvement, or primary site (organ) dysfunction plus bony lesions. The risk of bone metastasis is highest with prostate cancer.
Based on the disturbance in the bone remodeling, bone metastasis can be of three types,
The osteoclast is the bone-depleting cells that play a major role in the osteolytic type of bone metastasis. There is a decrease in the normal bone mass, and this type is more common in breast cancer.
The osteoblast is the bone-forming cells that are generated in excess by certain growth factors induced by cancer cells. Osteoblastic bone metastasis is a common sequela of prostate cancer.
The mixed type contains both bone formation and depletion and can spread from breast, stomach, and intestine cancers.
The two major types of cancer in the body are carcinoma (origin from the skin or lining that covers the organs) and sarcoma (origin from the connective tissue). Most bone metastasis is secondary to carcinoma.
The primary spread of bone metastasis is through blood, especially in the venous system. For example, breast and lung cancer mainly spread to the chest and the middle backbone region, while prostate cancer spreads to the hip and the lower backbone because of the common venous drainage. The primary development of bone metastasis occurs due to the interaction between the cancer cells from the primary site and the bone tissue cells resulting in the rise of numerous growth factors. These growth factors activate the bone cells in an abnormal fashion leading to metastatic cancer of the bone. Further, the entire process provides a suitable microenvironment for the disease progression.
Patients with bone metastasis often present with a number of signs and symptoms, like,
Pain and Tenderness: The pain is typically a dull type with gradual onset in the site of affected bones.
Nerve and Blood Vessel Compression: The pain related to nerve injury travels along the path of the affected nerves. Blood vessel compression may affect the associated structures with oxygen deprivation.
Elevated Calcium Level in the Blood: In the osteolytic type of bone metastasis, the tumor cells tend to stimulate the osteoclasts, which in turn causes excessive bone resorption and increased level of calcium in the blood (hypercalcemia). The patients will experience nausea, vomiting, constipation, and mental changes. This condition will confer a very poor prognosis of the existing health.
Fracture of the Weakened Bones: As there is excess bone loss, the bone architecture becomes so weak with an increased risk of fracture.
Spinal Cord Compression: When vertebral bones are affected by metastasis, the spinal cord may get compressed, which is one of the most significant complications that have to be addressed in an emergency. The symptoms included leg weakness, paraesthesia, and other neurological dysfunctions.
Early detection of bone metastasis helps in cancer staging and better the treatment outcome.
Radiographic analysis is the best choice for diagnosing bone metastasis. Instead of a single method, a combined imaging modality of the following technique will give an accurate diagnosis.
1) Plain X-Rays: The initial assessment of one pain in a cancer patient would be a plain X-ray. Pathological fractures or any osteolytic lesions of size more than two centimeters are well apparent in the plain radiograph.
Osteolytic bone metastasis will appear with poorly defined bone margins in the X-ray images.
While the mixed types of bone metastasis will show well-established modular features. The progress of the disease and their response to treatment can be monitored and maintained with plain X-rays.
2) Computed Tomography (CT): Computed tomography is an advanced radiographic technique that is more sensitive than plain radiographs. Also, CT can assess the density of the affected bone. The CT images are helpful in treatment planning and pre-surgical assessments.
3) Magnetic Resonance Imaging (MRI): Magnetic resonance imaging is highly sensitive and specific in diagnosing bone metastasis. Bone marrow involvement and spinal cord compression can also be detected earlier in MRI slices.
4) Bone Scan: A bone scan is also known as skeletal scintigraphy. The technique uses a radioactive substance to visualize the bone and assess its functions. The bone scan gives information about the whole body’s skeleton. Therefore, they are most useful in the early diagnosis of bone metastasis.
5) Single-Photon Emission Computerized Tomography (SPECT): A SPECT scan is also a type of nuclear imaging test that uses 99mTc-MDP radioisotopes to detect bone lesions. This process is more sensitive than bone scans.
6) Positron Emission Tomography (PET): PET is a nuclear medicine technique that measures the metabolic changes occurring in tissues or organs. In bone metastasis, the lesions are detected by the high glucose metabolism of the cancer cells.
7) Blood Tests: A complete blood count will show
Anemia (reduced red blood cells).
Thrombocytopenia (reduced platelets).
Calcium and alkaline phosphatase (enzyme) levels will be increased in the blood.
The goal of treatment in bone metastasis is to reduce pain, control the disease progression, and maintain the patient's quality of life.
Medications like painkillers and steroids are used for symptomatic relief. Bisphosphonates and Denosumab will help to prevent skeletal-related events associated with bone metastasis.
Radiotherapy is a standard protocol to attack cancer cells and will significantly reduce pain.
Chemotherapy to treat the primary tumor as well as the bone metastasis.
Surgical management may be necessary in case of fractures, single bone lesions (resection), and spinal cord compression.
Other therapies like bone-targeted radiopharmaceutical, radiofrequency ablation, cryoablation, and focused ultrasound are used in patients with recurrent pain.
Bone metastasis decreases the survival years in a patient with cancer. Early diagnosis and intervention can halt the progress of the disease. The treatment outcome largely depends on the severity of the condition, the extent of the spread, and the patient's other health condition. Unfortunately, most patients with bone metastasis do not survive for more than a year.