Thyroid neoplasms after therapeutic radiation for malignancies during childhood or adolescence

Recent data indicate that the risk of developing a thyroid neoplasm clearly is increased after high-dose, therapeutic radiation therapy during childhood. To better understand the time course, natural history, and histopathology of thyroid lesions that develop after high-dose irradiation, the authors undertook a retrospective study of all survivors of childhood and adolescent malignancies who were treated at Memorial Sloan-Kettering Cancer Center and who developed a clinically apparent thyroid neoplasm.

The authors searched the data base of the Department of Pediatrics, the hospital-based tumor registry, and the hospital medical records database for patients with thyroid neoplasms.

Thirty-three patients were identified who developed a thyroid neoplasm after therapeutic radiation. Primary diagnoses were Hodgkin disease (n = 18 patients), non-Hodgkin lymphoma (n = 10 patients), acute lymphoblastic leukemia (n = 2 patients), acute myeloid leukemia (n = 1 patient), Wilms tumor (n = 1 patient), and neuroblastoma (n = 1 patient). The median age at the time of diagnosis of the primary malignancy was 12.0 years (range, 3.7-18.3 years), the median radiation dose to the thyroid gland was 2400 centigrays (cGy; range, 1000-4200 cGy), and the median interval from the time of radiation therapy until the recognition of thyroid disease was 13.0 years (range, 6.2-30.1 years). Thirteen of 33 thyroid lesions (39%) were malignant (11 papillary carcinomas and 2 follicular carcinomas). Age at diagnosis, gender ratio, and time elapsed since initial treatment did not differ between patients with malignant and benign lesions, but the median radiation dose to the thyroid was lower in patients who had malignant disease compared with patients who had benign disease (2000 cGy vs. 2950 cGy; P = 0.03). Disease was confined to the neck in all patients who had malignant thyroid lesions; after a median follow-up of 6.5 years (range, 0.9-12 years), none of the patients developed progressive or recurrent disease.

Data from this study suggest that a high proportion of clinically apparent thyroid neoplasms that develop after therapeutic radiation for a childhood malignancy are malignant. However, most of these thyroid malignancies do not appear to behave in an aggressive fashion. Because thyroid neoplasms may not become evident for decades after radiation therapy, all individuals who are at risk require life-long follow-up.