Radioactive Iodine Therapy for Thyroid Malignancies

Excerpt

Thyroid cancer is the most common endocrine cancer and is histologically classified into 5 main types—papillary thyroid, follicular thyroid, oncocytic (Hürthle) cell, medullary thyroid, and anaplastic thyroid carcinoma. Among these malignancies, differentiated thyroid cancers—papillary, follicular, and oncocytic carcinomas—are the most common. Of these tumors, papillary thyroid cancer accounts for 80% to 90% of thyroid malignancies and has the best prognosis. The standard of care for differentiated thyroid cancers typically involves surgery, with or without postoperative radioactive iodine treatment.

Radioactive iodine targets and binds to the sodium-iodide symporter on cancer cells, allowing radioactive iodine to enter and accumulate within the cells. The radiation emitted then destroys malignant cells from within. Requirements and doses of radioactive iodine therapy vary based on individual risk factors.

The mechanism of action of radioactive iodine includes the following:

1. Selective uptake by thyroid cells: The thyroid gland is unique in its ability to efficiently absorb iodine, which it uses to produce thyroid hormones such as thyroxine (T₄) and triiodothyronine (T₃). This selective uptake is used in radioactive iodine therapy.

1. Iodine transporter: Thyroid cells have a specific transporter on their cell membranes called the sodium-iodide symporter, which actively transports iodine into the cells.

2. Radioactive iodine accumulation: During radioactive iodine therapy, patients are typically administered sodium iodide I 131 (¹³¹I), an isotope chemically identical to stable iodine. Thyroid cells take up this isotope in the same manner as nonradioactive iodine.

3. Local radiation: Once inside the thyroid cells, radioactive iodine emits β-particles, which are high-energy electrons. These β-particles damage the DNA of thyroid cells, leading to cell death.

4. Selective destruction of thyroid tissue: Radioactive iodine primarily targets thyroid tissue because thyroid cells absorb iodine more effectively than other body cells. This targeted approach for thyroid malignancies selectively destroys cancerous thyroid cells while sparing most other body tissues.

5. Systemic effects: Radioactive iodine has both local and systemic effects. A small amount of the unused isotope may be released from the thyroid gland into the bloodstream, where it can be absorbed by distant metastases or residual thyroid tissue not removed by surgery. This mechanism helps eliminate remaining cancer cells outside the thyroid gland.

A small amount of the unused isotope can be released from the thyroid gland into the bloodstream, where it may be taken up by distant metastases or residual thyroid tissue that was not removed during surgery.

Overall, the mechanism of action of radioactive iodine in treating thyroid malignancy involves exploiting the unique ability of thyroid cells to take up iodine and selectively destroy cancerous thyroid tissue through local and systemic radiation.

Epidemiology of Thyroid Cancer

Thyroid cancer is the most common endocrine cancer, ranking seventh in the world in terms of incidence, with a 5-year survival rate of 98.4%. Papillary (84%), follicular (4%), and oncocytic (2%) thyroid cancers arise from thyroid follicle cells and represent well-differentiated forms. More aggressive types, also derived from follicular cells, include poorly differentiated (5%) and anaplastic (around 1%) thyroid cancers. Medullary thyroid cancer (4%) is a neuroendocrine tumor that originates from the parafollicular cells of the thyroid. Women have nearly 4 times the likelihood of developing early-stage thyroid cancer compared to men, although underlying subclinical prevalence remains the same between sexes.

Clinical Applications of Radioactive Iodine Therapy

Clinical applications of radioactive iodine therapy fall into 2 main categories—initial treatment (ablation dose) after surgical thyroidectomy, which aims to destroy any thyroid tissue and cancer cells that remain after surgery, and subsequent treatments, which focus solely on residual or recurrent thyroid malignancy. Selecting the appropriate radioactive iodine therapy requires an accurate assessment of postoperative disease status, with serum thyroglobulin levels, neck sonography, and diagnostic iodine scanning as the most commonly used evaluation tools.