Comparative Analysis of the Growth Pattern of Thyroid Cancer in Young Patients Screened by Ultrasonography in Japan After a Nuclear Accident: The Fukushima Health Management Survey.

IMPORTANCE:Thyroid cancer generally grows at a very slow rate in adults, and overdiagnosis is a global issue. However, the detection of early-stage thyroid cancer by screening is not well described in young patients. To prevent overdiagnosis, it is essential to understand the natural course of thyroid cancer growth detection by ultrasonography screening in young patients. OBJECTIVE:To evaluate the natural progression of thyroid cancer in young patients. DESIGN, SETTING, AND PARTICIPANTS:An observational study evaluated changes in the diameter of malignant or suspected malignant thyroid tumors on 2 occasions. Changes in malignant thyroid tumor diameter were estimated during the observation period between the screening and confirmatory examinations in the first-round thyroid ultrasonography examination of the Fukushima Health Management Survey in patients younger than 21 years after a nuclear accident at a power plant in Fukushima, Japan. In total, 116 patients cytologically diagnosed with or suspected to have thyroid cancer were classified into 3 groups based on a greater than 10% reduction, a change of -10% to +10% in diameter, and a greater than 10% increase in tumor diameter. The association between tumor growth rate and tumor diameter was analyzed. The study was conducted from March 1, 2016, to August 6, 2017. MAIN OUTCOMES AND MEASURES:Tumor volume changes, the coefficient of growth of thyroid cancer in young patients, and the association between the observation period or tumor diameter and them. RESULTS:Of 116 patients, 77 were female; the mean age was 16.9 years (median, 17.5 years). The mean observation period was 0.488 (range, 0.077-1.632) years. No significant differences in age, sex, tumor diameter, observation period, or serum levels of thyrotropin and thyroglobulin were observed among the groups. Whereas tumor volume changes were not linearly correlated with the observation period (Pearson R = 0.121; 95% CI, -0.062 to 0.297), the coefficient of growth was significantly and negatively correlated with the tumor diameter in the screening examination (Spearman ρ = -0.183; 95% CI, -0.354 to -0.001), suggesting growth arrest after the initial proliferation phase. CONCLUSIONS AND RELEVANCE:Ultrasonography screening could reveal asymptomatic thyroid cancer that is falling into a growth arrest pattern in many young patients. Considering the long life expectancy, prevention of overdiagnosis necessitates careful long-term monitoring without immediate diagnosis for suspected noninvasive thyroid cancer.