Thyroid Cancer

Risk Factors

1. Cancer of Thyroid tissue
2. Cancer of tissues relating to Thyroid Gland such as medullary cell carcinoma

Thyroid tissue cancer can be divided into three groups as seen below:

1. Well differentiated thyroid cancer
2. Poorly differentiated thyroid cancer
3. Anaplastic cell carcinoma

These 3 groups have vastly different progression and severity. Here we will be covering Well-differentiated thyroid cancer, which is the most common type, accounting for 80-90% of all cases.

Well differentiated thyroid cancer can be further categorized into two minor groups called papillary cell carcinoma and Follicular cell carcinoma. Papillary cell carcinoma spreads through the lymphatic system; as such cancers can often spread to lymph node locates at the neck. Follicular cell carcinoma on the other hand spreads through the blood; as such it can often be found in the bone, lung, liver, and brain. Considering the vital organs, Follicular cell carcinoma is considered to pose a higher risk than Papillary cell carcinoma.

Diagnosis

Patients will experiences lumps at the throat referred to as thyroid nodules. If such lumps are found a doctor’s visit is recommended so that diagnosis can be provided. These tumors often develop slowly and do not have obvious symptoms; as such individuals with thyroid cancer may be unaware that they are affected until it has already spread. Diagnosis of thyroid cancer may include Thyroid hormone medication and gauging responses. An analysis of thyroid cells for example via biopsy, a Thyroid scan, and ultrasound can help to confirm diagnosis.

After diagnosis, surgery is considered the most effective treatment, these may include: partial thyroidectomy or total thyroidectomy (in cases where cancer has spread). Surgery may include removal of affected lymph nodes. Thyroid cancer of the papillary cell group has characteristics such as Multifoci, meaning that this cancer can occur in numerous locations. The size of the tumor may also be too insignificant to notice, however it may grow over a period of time. In principle, treatment will continue after surgery to destroy left over tissues that may contain cancer cells. A study conducted between patients who after surgery received radioactive iodine therapy for thyroid cancer and those who did not show that the first show a lower death rate and recurrence rate. It is important to note that patients do not necessarily require radioactive iodine therapy.

However, generally patients affected by Well-differentiated thyroid cancer will receive consistent radioactive iodine therapy.

Radioactive iodine is specific iodine that contains radioactive elements to be used in medical treatments. When radioactive iodine (RAI), also known as I-131, is taken into the body in liquid or capsule form, it concentrates in thyroid cells. The radiation can destroy the thyroid gland and any other thyroid cells (including cancer cells) that take up iodine, with little effect on the rest of your body. Due to radiation, use requires proper care and safety precautions. RAI is available as liquid and in capsule form, the difference in benefits are negligible, however despite capsules making for easy consumption, the drawback are the high costs associated with this method.

Once RAI enters the body it will absorbed into the tissues in the thyroid gland, excess radiation will be expelled through waste such as fecees, urine, or saliva (especially urine).

Prevention

Once RAI is absorbed it will provide beta and gamma rays. Beta rays will be able to move around slightly, it will provide rays into the thyroid tissues it passes through. This will result in oxidation which can remove any cancer cells that may still be present after surgical treatment.

Treatment

Following surgical treatment your doctor will determine whether continued treatment with RAI will be necessary. If RAI is chosen, your doctor will then determine whether to provide Low dose or High dose RAI. Patients that require low dose RAI will not have to stay at the hospital overnight. However, patients that require high dose RAI may have to spend 2-4 nights in the hospital. The reasons for sleeping over at the hospital are as follows:

1. Patient has received exposure to high levels of iodine radiation which may spread throughout the body. As such, to prevent exposing other people, a temporary quarantine is required.
2. At the hospital patients who receive Radiation sickness can be treated accordingly.

Patients will receive advice and recommendations upon discharge from the hospital as there may still be residual traces of radiation in their body (in low quantities). Patients will only be allowed to leave the hospital when radiation levels are deemed to not be harmful to the general public to minimize radiation exposure.

Side effects (RAI)

For low dose RAI (less than30 mCi) there are minimal side effects. Hospitalization usually only occurs when the patient experiences a lack of appetite or has salivary gland or thyroid infection. Other symptoms may include a low fever, laryngitis, nausea, and vomiting. These should subside within 1-2 days. Individuals may be worried that radiation exposure can lead to cancer in the long-term, however research has shown that dose of RAI (lower than 1,000 mCi) does not increase the chance of further cancer.

Diagnosis

• Total body scan (I-131)
• Results of Thyroglobulin test
• Ultrasound
• PET/CT Scan

1. Total body scan (I-131) or whole body scan is a test that checks for the return or spread of thyroid cancer. To prepare for a Total Body Scan, you will be asked to swallow a capsule or liquid that contains a very small amount of radioactive iodine (RAI). This will be absorbed by any remaining thyroid cells in your body. You will then be asked to return for the scan in about 48 hours. This involves lying down under a large camera that scans for x-rays being emitted by any remaining radioactive iodine that may have been captured in your body. If any thyroid or thyroid cancer cells are present, they may show up as spots on the x-ray film. However, if only microscopic thyroid cancer cells are present in the body, they are not always visible on the scan. For this test patients must not have received Thyroid hormone therapy for 4-6 weeks prior. Patients should also avoid consumption of iodine i.e. seafood and supplements 2 weeks prior to the tests.
2. Thyroglobulin(Tg) Thyroglobulin measurements and imaging play a key role in the follow-up of thyroid cancer. Thyroglobulin, a glycoprotein stored in the thyroid follicle, is released into the circulation in small quantities from both normal and malignant thyroid tissue. Following thyroidectomy and ablative I-131 treatment, circulating thyroglobulin becomes gradually undetectable, and a subsequent rise signals a recurrence. The serum thyroglobulin level is most sensitive when TSH is high, and it may be falsely negative if TSH is low, as in patients receiving replacement/suppressive thyroid hormone supplements. In the early period after thyroidectomy and initial I-. 131 ablation, serum thyroglobulin levels may be positive, despite the absence of persistent disease. They become undetectable over time without further treatment. Thus, increasing rather than stable or decreasing thyroglobulin levels following I-131 ablation are a better predictor of persistent disease.
3. Ultrasound– Ultrasound of the neck is another imaging option, used primarily at follow-up. It offers a relatively inexpensive means of monitoring patients at low risk, in whom recurrences are frequently limited to the lymph nodes.
4. PET/CT Scan is effective in diagnosis of more complicated cases especially in patients who have received RAI. For example if the results of Total body scan (I-131) is low, however if levels are still high, a PET/CT scan can help to provide more clarity on possibility that the cancer has spread or still remains.

Author

Dr. Samart Rajchadara
Nuclear Medicine Oncologist