Radioactive iodine (RAI) therapy is a targeted treatment for specific types of thyroid cancer, used after the thyroid gland has been surgically removed. This form of nuclear medicine is designed to destroy any remaining thyroid cells and cancerous tissue that could not be removed during surgery. This article provides an overview of RAI therapy’s success rates, how outcomes are measured, and the factors that influence its effectiveness.
The Mechanism of Radioactive Iodine Therapy
Radioactive iodine therapy operates on the biological principle that thyroid cells naturally absorb iodine. This function allows for a targeted delivery of radiation using Iodine-131 (I-131), a radioactive form of iodine administered orally as a capsule or liquid. Once ingested, I-131 is absorbed into the bloodstream and selectively accumulates in any remaining thyroid tissue, meaning other cells receive minimal radiation exposure.
The I-131 then releases radiation that damages the DNA of the thyroid cells and destroys them. This process eliminates microscopic cancer cells that may be lingering in the thyroid bed or have spread to nearby lymph nodes. For the therapy to be effective, patients need a high level of thyroid-stimulating hormone (TSH) in their blood, which encourages the thyroid cells to take up the iodine. This is achieved either by temporarily stopping thyroid hormone replacement medication or by receiving injections of a synthetic TSH called Thyrogen.
How Treatment Success Is Measured
A primary tool for measuring success is the serum thyroglobulin (Tg) level in the blood. Thyroglobulin is a protein produced only by thyroid cells, so after the thyroid is removed and RAI therapy is complete, its level should be undetectable. A detectable or rising Tg level can indicate remaining thyroid cancer cells.
Physicians also use imaging to verify the absence of cancerous tissue. A diagnostic whole-body scan using a tracer dose of radioactive iodine can detect areas where iodine is being absorbed, suggesting residual tissue or metastatic disease. The ultimate goal of treatment is to achieve “no evidence of disease” (NED), determined when blood tests show undetectable Tg levels and scans show no signs of iodine-avid tissue. Patients who reach this status are considered to have had a successful response to treatment.
Statistical Success Rates by Cancer Profile
The success of radioactive iodine therapy is linked to the cancer’s type and stage. The treatment is most effective for differentiated thyroid cancers, such as papillary and follicular thyroid cancer, because these cancer cells retain the ability to absorb iodine. For patients with these types of cancer, the prognosis is positive.
Papillary thyroid cancer is the most common form. When it is localized to the thyroid gland or nearby lymph nodes, RAI therapy following surgery contributes to high survival rates, with the 20-year survival rate being over 95%. Follicular thyroid cancer also responds well to RAI treatment, though rates can be slightly lower if there is extensive invasion into blood vessels.
The stage of the cancer at diagnosis is another determinant of the outcome. When differentiated thyroid cancer is confined to the neck area, the success rates are high. If the cancer has spread to distant parts of the body, such as the lungs or bones, the effectiveness of RAI therapy can decrease. While RAI can still be used to manage distant metastases, the five-year survival rate for distant-stage thyroid cancer is approximately 76%.
RAI therapy is not used for medullary or anaplastic thyroid cancers. Medullary thyroid cancer develops from a different type of cell (C-cells) that do not absorb iodine, making the treatment ineffective. Anaplastic thyroid cancer is an aggressive form where the cells are so undifferentiated they have lost their ability to take up iodine.
Variables Impacting Treatment Efficacy
Several variables can influence the effectiveness of radioactive iodine therapy. Patient-specific factors, such as age, play a role in prognosis; younger patients under 55 have better outcomes and a lower risk of recurrence compared to older patients.
The specifics of the treatment plan also have an impact. The dosage of radioactive iodine administered is selected based on a patient’s individual risk assessment, with higher doses used for more advanced cancers.
Preparation for the treatment is also a factor. For RAI to be effective, the cancerous cells must be “hungry” for iodine, which is achieved by following a low-iodine diet for one to two weeks before treatment. This diet involves avoiding foods high in iodine, such as:
- Iodized salt
- Dairy products
- Seafood
- Certain processed foods
Adherence to this diet helps deplete the body’s iodine stores, leading to increased uptake of the radioactive iodine by any remaining thyroid cells.
Post-Treatment Monitoring and Long-Term Outlook
Following radioactive iodine therapy, patients enter a phase of long-term monitoring to ensure the treatment was successful and to detect any potential recurrence. This follow-up care is a standard part of managing thyroid cancer and involves a regular schedule of appointments with an endocrinologist or oncologist. The frequency of these visits is higher in the first few years after treatment and then may become less frequent over time.
The monitoring process includes periodic thyroglobulin (Tg) blood tests and neck ultrasounds to visually inspect the area for any abnormal tissue growth. For most patients treated for differentiated thyroid cancer, the long-term outlook is positive. Recurrence is uncommon and is often detectable early through routine monitoring. In many cases, recurrent disease can be effectively treated with additional therapies, which may include another round of radioactive iodine or other targeted treatments.