Natural history of thyroid cancer suggests beginning of the overdiagnosis of juvenile thyroid cancer in the United States.

Abstract

In a recent article, Bernier et al reported the increasing incidence of thyroid cancer among individuals aged <19 years in the United States, and concluded that such an increase is unlikely to be explained entirely by increases in medical surveillance.1 Based on recent data concerning the natural history of thyroid cancer, I herein express my concern regarding overdiagnosis.2 Cancer cells are believed to be generated from benign tumor cells, and it has been recognized that when cancer cells are left untreated, they become more and more aggressive. However, data from some recent studies have suggested that the natural history of juvenile thyroid cancer is different from what we have believed for many years. Data from the Fukushima Health Management Survey (FHMS) demonstrated that with ultrasonography, small thyroid cancers can be detected at a high prevalence even in children, and the majority of these cancers are likely to stop proliferation after some decades.3, 4 Another study supports this assumption. During observation, most papillary microcarcinomas stop their growth, and some even shrink.5 Because to my knowledge no effects of radiation on health have been reported in Fukushima, most of the cases found in the FHMS are likely to be overdiagnosed. Therefore, the majority of patients will demonstrate no symptoms due to thyroid cancer throughout their life. It is interesting to note that even though nearly all cases showed a small tumor size because they were diagnosed with ultrasonography, >80% demonstrated extrathyroidal extension of cancer cells at the time of surgery.3 These data suggest that, in patients with juvenile thyroid cancer, even cases diagnosed with metastasis can be a cause of overdiagnosis because cancer cells are likely to stop proliferation after metastasis. When reading the study by Bernier et al, one should take into account this self-limiting characteristics of juvenile thyroid cancer. In the FHMS, >100 children among 300,000 were diagnosed with thyroid cancer after ultrasonography screening, and the estimated age-standardized incidence was approximately 70 times greater than that in the United States before 2000.1, 3 It is clear that there is a large reservoir of clinically silent thyroid cancers in the pediatric population. Furthermore, 3 children with distant metastasis were identified incidentally on ultrasonography screening. Although the number was extremely small, the roughly estimated age-standardized incidence was approximately 10 cases per 1 million, and was approximately 50 times greater than that in the United States before 2000. It is important to realize that these cases were detected incidentally on ultrasonography screening, and therefore we cannot exclude the possibility that even some of these cases were overdiagnosed. The authors should assess data regarding the number of thyroid ultrasounds for children administered during the same period and in the same area because, as was the case in Korea, this number is likely to be positively correlated with the number of overdiagnosed cases. When an increasing incidence of juvenile thyroid cancer is observed, priority should be given to the prevention of overdiagnosis over the search for the reason for the true increase. Children diagnosed with thyroid cancer are likely to experience the disease not only physiologically but also psychologically, socially, and economically. Therefore, the harm of overdiagnosing thyroid cancer in the young is more serious and complicated compared with in adults, and it is not only a health problem but also an issue of human rights. The early detection of thyroid cancer in children is not likely to be beneficial because its prognosis is extremely favorable. I hope that physicians in the United States will carefully consider the increase in both the incidence of juvenile thyroid cancer and the number of thyroid ultrasonography tests to prevent the expansion of overdiagnosis. No specific funding was disclosed. The author made no disclosures.