Thyroid Cancer Metastasis to Bone (Symptoms, Diagnosis, and Treatment)

What is Bone Metastasis from Thyroid Cancer?

Bone metastasis in thyroid cancer means that cancer cells have broken away from the original site of the tumor (i.e., the thyroid) and have spread to the bones. The bones are the second most common site for thyroid cancer to spread after the lungs. This type of metastasis occurs in about 2 to 15 percent of all types of thyroid cancer. Thyroid cancer cells usually travel to the bones through the bloodstream and grow there. Spread through the lymphatic system often involves the lymph nodes and does not play a major role in the development of bone metastases. Direct spread to the bones is also very rare in thyroid cancer.

Stages of Thyroid Cancer Bone Metastasis

Thyroid cancer bone metastasis is a complex process that involves several stages:

• Local invasion: Thyroid cancer cells break away from the original tumor and invade nearby tissues, including nearby blood or lymphatic vessels.
• Metastasis: These cells travel through the bloodstream (the main route for thyroid cancer bone metastasis) or the lymphatic system to distant organs and tissues, such as bones.
• Attachment: Cancer cells adhere to the inner walls of blood vessels in the bone area. These cells may also secrete signaling molecules that attract them to a suitable environment for growth in bone tissue.
• Bone tissue invasion: Cancer cells invade bone tissue and disrupt bone structure and function. These cells also stimulate the production of osteoclasts. Osteoclasts are cells responsible for the resorption and breakdown of bone tissue. During this process, known as osteolysis, the natural balance between bone formation and destruction is disrupted and can cause the bone to weaken or break down.
• Proliferation: Cancer cells settle in bone tissue and proliferate, forming metastatic tumors. These tumors weaken the bone structure and may cause symptoms such as pain, fractures, and other complications.

Note: Cancer cells continuously interact with the bone microenvironment from the moment they enter the bone tissue and throughout this process. The bone microenvironment includes bone cells (osteoblasts and osteoclasts), immune cells, and the extracellular matrix. This continuous interaction leads to further tumor growth and makes it resistant to treatment.

Prevalence of bone metastasis in thyroid cancer types

Between 2 and 15% of patients with thyroid cancer develop bone metastasis. The most common type of thyroid cancer is papillary carcinoma, which accounts for about 85% of all cases. The probability of spreading to bone is relatively low and has been reported to range from 1 to 7% according to different sources. In contrast, follicular thyroid carcinoma, which accounts for less than 15% of all differentiated thyroid cancers, has a much higher probability of spreading to bone, and the incidence of bone metastasis in it has been estimated to range from 7 to 28%. In addition to these two types, bone metastasis in medullary thyroid carcinoma (MTC) is also observed in 16 to 19% of cases during the follow-up period of patients. This indicates that bone involvement due to thyroid cancer is not limited to a specific type of thyroid cancer and its rate varies significantly depending on the histological type of the tumor.

Symptoms

The most common symptom of bone metastasis from thyroid cancer is bone pain, which usually becomes more severe as the disease progresses. Also, because metastatic tumors weaken the bone structure, patients are at increased risk for bone fractures; sometimes they break even with minimal impact or pressure. In addition to these two main symptoms, bone metastasis is also accompanied by other symptoms; These include:

• Difficulty walking or moving due to bone weakness or instability
• Swelling or deformity in the affected bones
• Numbness or tingling in the limbs
• Limited range of motion in joints near the affected bones
• General fatigue and weakness, caused by the body’s response to the spread of the cancer
• Increased risk of secondary infections in the weakened bones
• Compression of nearby nerves or blood vessels and resulting neurological symptoms
• Changes in posture due to bone deformity or spinal involvement
• Increased blood calcium levels (hypercalcemia) and resulting symptoms such as intense thirst, frequent urination, nausea, constipation, drowsiness, or confusion
• Compression of the spinal cord and resulting symptoms such as back pain, numbness or weakness in the legs, numbness in the abdomen, and loss of bladder or bowel control
• Unexplained weight loss
• Worsening bone pain At night or while resting

These symptoms have a significant impact on the patient’s quality of life and occur gradually or suddenly, depending on the location and severity of bone involvement.

Diagnosis of thyroid cancer bone metastasis

Diagnosis of bone metastasis from thyroid cancer is usually performed with a combination of several methods, including:

1. Bone scan: This method is used to examine the condition of the bones and identify abnormal areas. In a bone scan, a special substance is injected into the body. This substance collects in parts of the bone with abnormal activity. These areas can be a sign of a lesion or the spread of cancer to the bone.

. CT Scan: CT SCAN uses X-rays to provide the doctor with detailed, multi-layered images of the bones and surrounding tissues. These images show structural changes in the bone, bone lesions, or other damage that may have been caused by the spread of cancer. It also helps the doctor better assess the extent of the disease.

3. 3. MRI: This method uses a magnetic field and radio waves to provide detailed images of the body, which is useful for examining the soft tissues around the bone, bone marrow, and more accurately diagnosing the site of involvement. MRI can identify even small changes in or around the bone that may have been caused by metastasis.
4. 4. Bone biopsy: Sometimes the doctor performs a biopsy to confirm the diagnosis of bone metastasis. In this method, a small amount of bone tissue is removed and examined in the laboratory under a microscope. The presence of cancer cells in this sample can confirm the diagnosis of bone metastasis.
5. 5. PET Scan: PET SCAN is one of the advanced imaging methods to check for the presence of cancer cells in the body. In this method, a small amount of sugar with a small amount of radioactive material added is injected into the body. Cancer cells usually consume more sugar due to their greater growth and activity compared to healthy cells; therefore, this material accumulates more in the parts where cancer cells are active. After the injection, a special PET device takes images of the body and shows areas with abnormal activity. This method is effective in identifying small tumors or areas of cancer spread that may not be seen in other imaging methods.
6. 6. Supplementary tests: In some cases, in addition to examining the bones, other tests such as blood tests and checking the levels of some markers related to thyroid cancer are also performed. These tests, along with the imaging results, help to better diagnose the condition of the disease, the extent of cancer spread, and choose the best treatment method.

Treatment methods

Treatment of bone metastases involves several different approaches. The doctor will choose the best treatment method based on the patient’s condition. Note that there is no definitive cure for bone metastases; however, a combination of the following methods can prevent the progression of the disease, significantly reduce symptoms, and improve the patient’s quality of life.

Surgery

The main treatment for thyroid cancer metastases to bone is a total thyroidectomy (complete removal of the thyroid gland). The main goal of this surgery is to remove as much tumor tissue as possible from the body, which is performed by a thyroid surgeon. In cases where the involved bone is at risk of fracture, a team consisting of a thyroid surgeon and an orthopedic surgeon may, in addition to removing the tumor, also stabilize the involved bone (strengthen the structure of the same bone that has metastasized). This procedure usually reduces symptoms, reduces the risk of fracture, and improves bone health. The surgeon’s experience and skill play a decisive role in the success of the procedure and reduces possible complications. Also, using the latest facilities and equipment during surgery increases the accuracy of the operation and significantly reduces the risks of surgery. For this reason, choosing an experienced surgical team equipped with modern technologies is of great importance in the final result of the treatment.

Radioactive iodine treatment

Since normal thyroid cells and some thyroid cancer cells (especially differentiated types) have the ability to absorb iodine, this feature is used for treatment with radioactive iodine (Iodine-131). After entering the body, Iodine-131 accumulates in cancer cells capable of absorbing iodine, including bone metastases, and helps destroy these cells and control tumor growth by irradiating them. This method is also used in the treatment of thyroid tissue remaining after surgery; however, higher therapeutic doses are used to treat bone metastases caused by thyroid cancer. However, this method is only effective in patients whose lesions retain the ability to absorb iodine (iodine-loving or iodine-absorbing). Some bone metastases, especially in advanced stages of the disease, gradually lose the ability to absorb iodine, and as a result, the effectiveness of radioactive iodine treatment is reduced. For this reason, before administering radioactive iodine, the doctor usually evaluates the presence of iodine uptake in the lesions using appropriate imaging.

Radiation therapy

Radiotherapy targets and destroys cancer cells in the bone. This method can be given as external beam radiation therapy or internal radiation therapy (brachytherapy). Radiation therapy reduces pain, shrinks the size of the tumor, and prevents the cancer from spreading further to surrounding tissues.

Internal radiotherapy (radiopharmaceuticals)

In this method, a radioactive substance is injected into the patient’s bloodstream and accumulates in the bone tissue affected by cancer. The substance then destroys the cancer cells by releasing radiation from within the tissue. The advantage of this method over many other treatments is its relatively fewer side effects.

Local tumor ablation

This method is a minimally invasive technique in which, with the help of CT scan imaging, a special probe is guided precisely to the site of the bone tumor. Then, by applying intense heat, extreme cold, electric current, or special chemicals, the tumor tissue is destroyed from within. This option is especially useful for patients who are not physically fit for open surgery.

Thyroxine Suppression Dose

Thyroid hormone (thyroxine) is used in high doses as a common adjuvant treatment for differentiated thyroid cancer. Since thyroid cancer cells have receptors for thyroid-stimulating hormone (TSH), which can stimulate their growth, doctors suppress the body’s TSH levels by administering high doses of thyroxine to prevent further stimulation of the cancer cells and the possibility of recurrence or spread of the disease. However, it is important to note that this method does not directly eliminate existing bone metastases and only has a preventive and control role. Also, long-term use of high doses of thyroxine can increase the risk of bone loss and osteoporosis. For this reason, doctors usually carefully adjust the amount of TSH suppression according to the patient’s risk level and the duration of treatment.

Systemic (targeted) therapy

Targeted therapy focuses on the specific changes and characteristics of cancer cells that cause them to grow and spread. Doctors usually use this type of therapy when bone metastases are extensive or have spread to other organs; otherwise, the role of systemic therapy is very limited.

Bisphosphonates and denosumab

These drugs help strengthen bones and reduce the risk of fractures by inhibiting the process of bone resorption. Doctors usually prescribe these drugs along with other treatments to manage the complications of bone metastases.

Pain management

Pain control through pain medications, physical therapy, and nerve blocks is essential to improve patients’ quality of life. Complementary therapies such as acupuncture or massage also provide additional relief and increase the patient’s sense of comfort.

Survival rates for bone metastases from thyroid cancer

Survival rates for bone metastases from thyroid cancer vary depending on factors such as the stage of the cancer, the patient’s health status, and the extent of the metastases. When differentiated thyroid cancer (DTC) metastasizes to the bone, the patient’s chances of survival decrease significantly. For example, from the moment bone metastases are first diagnosed, about 93% of patients survive for one year, 89% for two years, 80% for three years, 61% for five years, and only 27% for ten years. The severity of this decrease in survival does not depend solely on the presence of bone metastases; its association with involvement of other parts of the body also plays an important role in worsening the patient’s condition. In a large population study of more than 64,000 patients with thyroid cancer, it was found that patients with only bone involvement survived an average of 66 months. This number decreased to 14 months for patients with metastases to one site in addition to bone, and to 6 months for those with multiple sites.

The Bottom Line

The incidence of thyroid cancer is increasing, with more than 567,000 new cases reported annually. Bone metastases from thyroid cancer are the second most common site of systemic spread of the disease after the lung. About 2 to 15 percent of all thyroid cancers result in bone metastases. Follicular thyroid cancer is also more likely to spread to bone than papillary thyroid cancer.