Radiation-induced thyroid cancer

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Radiation-induced thyroid cancer is an important etiology of thyroid cancer.

Epidemiology

Information about radiation-induced thyroid malignancies comes from several long-term cohort studies along with some case-controlled studies and their subsequent meta-analyzes. Although these studies vary in terms of their inclusion criteria, trends across most studies remain similar.

There are three essential patient groups in which radiation-induced thyroid cancer occurs.

Atomic bomb survivors (Japan)

Childhood exposure to radiation had a relative risk of thyroid cancer up to 1.7-18.9. Children under the age of one had a relative risk of 237 compared to children who were not exposed to radiation . Although substantial, the incidence of thyroid cancer in non-exposed children was negligible, contributing to high relative risk. Consumption of potassium iodide was found to be a protective factor.

Clinical radiation exposure in children

Increased risk of 390/100,00 excess thyroid malignancies in children undergoing diagnostic CT scans. The average dose is estimated at 0.6-8.7 mGy .

Clinical radiation exposure in adults

Very few studies examining the effect of radiation on thyroid malignancies have been reported. The risk in adults was reported as being significantly less in adults compared to children. In a study examining lymphoma patients that underwent radiation treatment, abnormal USS images were found in 44% of patients, and 50% of patients demonstrated abnormal TFTs .

Pathology

The thyroid gland is one of the most radiosensitive organs in the human body. The most common cancer in patients being exposed to radiation is papillary thyroid carcinoma (94% compared to 80% in unexposed patients). RET/PTC rearrangements and BRAF mutations were commonly observed .

Treatment and prognosis

There appears to be a lack in the literature surrounding screening for thyroid malignancies following radiation exposure. Some authors suggest screening with TSH annually in patients who have had high dose exposure (≥25 Gy). Surveillance with physical examinations is also recommended.

There is no data advocating or against screening with ultrasound scans, and hence a risk-benefit decision must be made by the treating clinician with regards to unnecessary anxiety around benign appearing nodules and high dose exposure (≥100 Gy).