Radiation therapy for thyroid cancer

Radiation therapy uses high-energy rays or particles to destroy cancer cells.

Many people with thyroid cancer have radiation therapy. Your healthcare team will use what they know about the cancer and about your health to plan the type and amount of radiation, and when and how it is given.

You could have radiation therapy to:

• destroy cancer cells left behind after surgery to reduce the risk of the cancer coming back, or recurring (called adjuvant therapy)
• destroy any normal thyroid tissue remaining after surgery to reduce the risk of the cancer coming back (called thyroid remnant ablation)
• treat cancer that has spread to lymph nodes or other parts of the body
• relieve pain or control the symptoms of advanced thyroid cancer (called palliative therapy)

The following types of radiation therapy are most commonly used to treat thyroid cancer.

Radioactive iodine (RAI) therapy is a type of internal radiation therapy. The radioactive material is called radioactive iodine, or I-131. It is given by mouth and travels to reach cells all over the body. Cancer cells take up the radioactive material, which destroys them.

RAI therapy is usually given after surgery for thyroid cancer. It doesn't work for all types of thyroid cancer because some cancer cells don't absorb iodine. You may be given a small test dose of I-131 to see if the cells absorb iodine. RAI therapy is used most often for papillary and follicular (including Hurthle cell) thyroid cancers.

Radioactive iodine therapy may be offered if the:

• tumour is larger than 4 cm
• tumour has grown through the thyroid and spread to nearby tissues and structures
• cancer has spread to many lymph nodes in the neck
• cancer is an aggressive variant of papillary or follicular (including Hurthle cell) thyroid cancer, is a rare type of thyroid cancer or has other high-risk features when seen under a microscope
• cancer comes back in the same place or close to where it started (called a local or regional recurrence)
• cancer has spread to other parts of the body (called distant metastasis)

The normal amount of iodine in the body needs to be lowered before RAI therapy starts. This will help make sure that the thyroid cancer cells absorb the radioactive iodine. When there is less iodine in the body, the pituitary gland makes more thyroid-stimulating hormone (TSH) and releases it into the bloodstream. High TSH levels will make any thyroid tissue and thyroid cancer cells absorb radioactive iodine.

People who have had a thyroidectomy to treat thyroid cancer usually have to take hormone therapy with levothyroxine (Synthroid, Eltroxin). This drug replaces thyroxine, which is a hormone that would normally be made by the thyroid. It also lowers the amount of TSH in the body. To increase TSH levels, you may be asked to stop taking levothyroxine for several weeks before you start RAI therapy.

Another way to increase TSH levels is to give a drug called recombinant TSH (Thyrogen). It is an artificial form of TSH that is given by injection into muscle usually once daily for 2 days before RAI therapy.

You may also be told to follow a low iodine diet for 1 to 2 weeks before RAI treatment. You should avoid or limit salt and salty foods, milk and milk products and seafood. Ask your healthcare team for a complete list of foods to avoid or limit.

Your body gives off small amounts of radiation during and for some time after the treatment. Once the dose of radioactive iodine is given, you will need to stay away from people for a few days. This may be done by staying in a private room in the hospital or treatment centre or by following special precautions at home. Talk to your healthcare team about safety precautions you will need to take while in the hospital or at home to protect others from exposure to radiation.

A radioactive iodine scan is a nuclear medicine imaging test that uses radioactive material to examine the thyroid and look for any abnormal areas or thyroid cancer cells in other parts of the body. It is usually done about 1 week after RAI therapy. It is also repeated 6 to 12 months after RAI therapy to check how the cancer responded to the treatment and if there are any cancer cells remaining. You may need to have more RAI therapy if cancer cells are found with the scan.

Blood tests are also done to check if you need more treatment. The healthcare team will measure the levels of thyroglobulin (Tg) and TSH in your blood. Abnormal levels may mean that cancer cells were left behind after radioactive iodine therapy or that the thyroid cancer has recurred. An ultrasound of the neck may be done along with the blood tests.

During external radiation therapy, a machine directs radiation through the skin to the tumour and some of the tissue around it. External radiation therapy is also called external beam radiation therapy.

External radiation therapy may be used to treat:

• medullary and anaplastic thyroid cancers
• thyroid cancers that don't take up iodine
• thyroid cancers that have spread beyond the thyroid and to lower the chance of cancer coming back after surgery
• pain or symptoms of advanced thyroid cancer that has spread (metastasized) or that can't be removed with surgery (unresectable)

How long and how often external radiation therapy is given depends on many factors including the type of thyroid cancer, the size of the area being treated and the goal of treatment. External radiation therapy is usually given once a day, 5 days a week. Smaller doses of radiation therapy may be given 2 times a day (called hyperfractionation) for anaplastic thyroid cancer.

Doctors may use the following external radiation techniques to accurately target the area to be treated and spare as much surrounding normal tissue as possible.

3D conformal radiation therapy (3D-CRT) directs many beams of radiation at the tumour. The radiation oncologist uses a CT scan or an MRI to map the exact location and shape of the tumour. The radiation beams are then shaped and aimed at the tumour from different directions to treat the tumour from all angles. Each individual beam is fairly weak and less likely to damage normal tissue. A higher dose of radiation is delivered where the beams meet at the tumour.

Intensity-modulated radiation therapy (IMRT) is similar to 3D-CRT in that it delivers radiation from many different angles to treat the entire tumour. In addition to shaping and aiming the radiation beams, IMRT allows the radiation oncologist to adjust the strength (intensity) of the individual beams. This reduces the dose of radiation reaching nearby normal tissue while allowing a higher dose to be delivered to the tumour. It is useful for treating tumours in hard-to-reach areas.

During radiation therapy, your healthcare team protects healthy cells in the treatment area as much as possible. Side effects of radiation therapy will depend mainly on the type of radiation therapy used, the size of the area being treated, the specific area or organs being treated, the total dose of radiation and the treatment schedule. Tell your healthcare team if you have side effects that you think are from radiation therapy. The sooner you tell them of any problems, the sooner they can suggest ways to help you deal with them.

These are common side effects of RAI therapy for thyroid cancer:

• nausea and vomiting
• taste changes
• dry mouth
• swelling in the neck
• sore throat
• watery eyes
• low sperm counts in men
• changes to menstrual periods in women

These are common side effects of external radiation therapy for thyroid cancer:

• skin problems, such as red, irritated and peeling skin
• fatigue
• dry mouth and thick saliva (spit)
• hoarseness
• swallowing problems, including pain during swallowing
• weight loss

Find out more about radiation therapy and side effects of radiation therapy. To make the decisions that are right for you, ask your healthcare team questions about radiation therapy.