Risk of second malignancies among survivors of pediatric thyroid cancer.

Background: Thyroid carcinoma is a very rare tumor in the pediatric age group, accounting for only 1.5-3% of childhood carcinomas in the United States and Europe. We aimed to identify the risk of a second malignancy among pediatric thyroid cancer survivors.

Methods: The cohort analysis consisted of pediatric cancer patients aged less than 20 years, diagnosed with a primary thyroid cancer, identified by site code ICD-0-3: C739, and reported to the SEER 9 database between 1973 and 2013. They were followed up until December 31, 2013; the end of the study period, or up to death if earlier.

Results: Out of 1769 patients diagnosed primarily with thyroid carcinoma, 42 patients had a total of 45 incidences of subsequent malignancies. The mean age of patients at the initial diagnosis of thyroid cancer was 16 years. Females (90.5%) had a significantly higher incidence of second malignancies (SM) than males (9.5%). The overall Standardized Incidence Ratio (SIR) of SM in the study patients was higher than expected (SIR = 1.48). Some specific sites showed significantly higher incidences: the salivary glands (SIR = 33.95), the gum and other parts of the mouth [excluding the lips, tongue, salivary glands and floor of the mouth] (SIR = 24.53)*** and the kidneys (SIR = 5.72). The overall risk of SM in patients who had received radioactive iodine was higher than expected (SIR = 4.41). The cumulative incidence of SM after treatment of thyroid cancer in children increases steadily over 40 years (11.92%).

Conclusions: Race, gender, histological subtypes, and radioactive iodine are potentially significant prognostic factors for the development of SM among pediatric thyroid cancer survivors. Identification of underlying mechanisms that raise the risk of SM is important for both treatment and follow-up strategies.