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Association between screening-detected thyroid cancers, radiation exposure ‘unlikely’ in Fukushima children

Many thyroid cancers detected by ultrasound screening in Japanese children in the 5 years after the 2011 Fukushima Daiichi nuclear accident mirror many low-risk, sporadic cases of adult thyroid cancer and are most likely not associated with radiation exposure, according to findings published in JAMA Otolaryngology Head & Neck Surgery.

“Large-scale mass screening of young people using ultrasonography resulted in the diagnosis of a number of thyroid cancers, including potential overdiagnosis cases,” Akira Ohtsuru, MD, PhD, professor in the department of radiation health management at Fukushima Medical University, Japan, told Endocrine Today. “These findings indicate that ultrasonography screening can identify many detectable cancers from large pool of nonclinical and subclinical thyroid cancers among individuals of a relatively young age, in an age-dependent manner.”

The level of radiation exposure in Fukushima immediately after the great earthquake in eastern Japan on March 11, 2011, has been deemed to be much lower than the levels reported in Chernobyl immediately after the 1986 nuclear power station accident, Ohtsuru and colleagues wrote in the study background. However, the researchers noted that there was a divergence in estimations of the thyroid equivalent dose of radiation during the early phase after the accident, as there was little direct measurement of the individuals, whereas radiation-induced thyroid cancers have been rated as causing some of the greatest concern after the accident.

“Thus, health surveillance, including thyroid screening, has been thought to be necessary for both scientific and social reasons,” the researchers wrote.

In an observational study, Ohtsuru and colleagues analyzed data from 324,301 Fukushima residents aged 18 years or younger at the time of the nuclear accident. Researchers assessed the number of detected cases of thyroid cancer during first (fiscal years 2011-2013) and second (fiscal years 2014-2015) rounds of screening, with age groups stratified by 3-year intervals. Researchers compared results against an age-specific incidence of unscreened cancers form a national cancer registry.

Researchers found that, among 299,905 participants screened in the first round (50.5% boys and men; mean age at screening, 15 years), 116 were diagnosed with malignant or suspected thyroid cancer. Among 271,083 participants screened in the second round, (50.4% boys and men; mean age at screening, 13 years), 71 were diagnosed with malignant or suspected thyroid cancer, according to researchers. In both the first- and second-round examinations, mean age at diagnosis was 17 years; however, mean tumor diameter was larger in the first- vs. the second-round screenings (mean, 12.7 mm vs. 9.7).

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Papillary thyroid cancer was the most common pathologic diagnosis, they wrote, at 149 of 152 cases (98%). The researchers also noted that the distribution pattern by age group at the time of the accident increased with older age in both screening rounds, with an interval between screenings of about 2.1 years. The number of detected thyroid cancer cases per 100,000 persons was estimated to be approximately 175 for those aged 18 years at the time of the accident in the first round of screening and 97 for those aged 18 years at the time of the accident during the second round of screening.

Researchers calculated that the estimated age-conditional incidence rate of cases per 100,000 person-years was 0.5 for those aged 15 to 17 years, 1 for those aged 18 to 20 years and 1.7 for those aged 21 to 22 years.

“Although a longer observation period is needed, this pattern by age at the time of the accident differs from that of the Chernobyl nuclear power station accident; for example, there was a higher frequency of cases of cancer at younger ages with a relatively short latent period after the Chernobyl nuclear power station accident,” the researchers wrote. “Thus, an association between the large number of thyroid cancers detected and radiation exposure is thought to be very unlikely, in addition to the very low doses of radiation in the Fukushima nuclear power station accident.”

Ohtsuru added that, to ensure the merit of early detection and prevent overdiagnosis via large-scale screening, development of a careful monitoring system is urgently needed based on the understanding of the natural history of thyroid cancer.

In commentary accompanying the study, Andrew J. Bauer, MD, medical director of the Pediatric Thyroid Center at Children’s Hospital of Philadelphia, and Louise Davies, MD, MS, research fellowship director of the VA Outcomes Group at the Veterans Affairs Medical Center in White River Junction, Vermont, wrote that researchers should not assume that every asymptomatically discovered case of pediatric thyroid cancer is a “triumph of early detection.”

“Going forward, research to better define which pediatric thyroid cancers are likely to progress, and a better understanding of the trajectory of the natural history of thyroid cancers detected in early life, is urgently needed,” Bauer and Davies wrote. “The extremely high prevalence of subclinical thyroid cancer shown by the Fukushima Survey, including many with lymph node metastases, suggests that we have much to learn about the natural history of lymph node metastases, but that we should not assume that every asymptomatically identified case of pediatric thyroid cancer is an instance of overdiagnosis.” – by Regina Schaffer

For more information:

Akira Ohtsuru, MD, PhD, can be reached at Fukushima Medical University, Department of Radiation Health Management, 1 Hikarigaoka, Fukushima 960-1295, Japan; email: ohtsuru@fmu.ac.jp.

Disclosures: One of the authors reports he received grants from commercial sponsor JCR Pharmaceuticals Co. Bauer and Davies report receiving travel funds from the International Agency for Research on Cancer.