A total of 166 children in Fukushima Prefecture had been either diagnosed with thyroid cancer or with suspected cases of cancer by the end of 2015 through screening conducted by the Fukushima Prefectural Government, following the 2011 nuclear plant crisis.
The figure is several dozen times higher than the estimated number of thyroid cancer patients based on national statistics, according to a panel of experts with the prefectural government. While the panel and the Environment Ministry say the effects of radiation in these cancer cases are unlikely, opinions are divided among experts about the causes of such a high occurrence rate of cancer in children.
"Compared to the estimated prevalence rates based on the country's statistics on cancer, which are shown in data including regional cancer registration, the level of thyroid cancer detection is several dozen times higher (in children of Fukushima Prefecture)," said the final draft for the interim report compiled by the prefectural government's expert panel on Feb. 15.
Most experts of epidemiology agree on the view that the number of thyroid cancer cases is high among over 300,000 targets in health checkups that started six months after the nuclear meltdowns.
A research team led by Shoichiro Tsugane, head of the Research Center for Cancer Prevention and Screening of the National Cancer Center and a member of the Fukushima government's expert panel, published a research paper on the matter in January this year and another team headed by Okayama University professor Toshihide Tsuda also published their paper in October 2015. While their calculation methods differ, the two teams both concluded that the number of cancer cases found in Fukushima children was "about 30 times" that of national levels.
There has never been an attempt in Japan to check thyroid cancer among hundreds of thousands of children who are not self-aware about symptoms such as lumps. Because of this, some experts pointed out earlier that the screening detected cancer in advance in those who may develop the disease later, and as a result, the number of cancer patients spiked temporarily. While such a rapid increase in the number of patients by early detection has been reported in other types of cancer, the figure remains as high as "several times higher than national levels." Tsugane and Tsuda both agree that the "30 times higher (than the national occurrence rates)" is unexplainable.
At the moment, the most likely theories for such a high rate of cancer detection are the "overdiagnosis theory" held by Tsugane's team and the "radiation effect theory" that Tsuda's team supports.
Overdiagnosis refers to the diagnosis of cancer by detecting hidden cancer cells that are not harmful even if left untreated.
The concept of cancer overdiagnosis has been argued for decades in areas including the lungs, chest and prostate, and its negative effects on cancer screening takers' physical and psychological conditions have been pointed out as a problem. In 2004, the Ministry of Health, Labor and Welfare canceled examinations for neuroblastoma, a type of pediatric cancer, saying that the test would impose large disadvantages on screening subjects due to overdiagnosis.
In South Korea, thyroid cancer screening has been rigorously carried out from the late 1990s targeting adults, and as a result, the number of thyroid cancer patients spiked 15 times. In the meantime, thyroid cancer death rates have not changed, which has been interpreted in a way that non-harmful cancer was detected in the screening process.
While the Fukushima screening mostly targets children, Tsugane argues that it's rational to judge that the reason behind such a high prevalence is overdiagnosis as seen in South Korea's studies, on the grounds that the maximum amount of radiation exposure in the thyroids of children in Fukushima Prefecture is estimated to be several dozen millisieverts, which is not enough to cause an increase of 30 times in the number of patients. He also argues that it appears that no phenomenon has been reported where the number of patients becomes higher in areas with high radiation levels. The prefectural government shares his opinion on the matter.
At the same time, Tsugane is not completely denying the effects of radiation in children's cancer, saying, "It would not be strange if a small portion of cancer cases was caused by radiation exposure, but we do not know the precise percentage."
Tsuda, on the other hand, took the difference in the timing of screening among children into account and argues that radiation exposure is the main cause of the high prevalence of cancer in children, saying that the occurrence rate is 4.6 times higher in Futaba County near the crippled nuclear plant compared to the city of Sukagawa and other areas that are farther from the power plant.
He does not deny the possibility of overdiagnosis, but because the spread of cancer cells to lymph nodes and other tissues could be seen in 92 percent of patients, Tsuda believes that overdiagnosis makes up 8 percent of the patients at most.
In addition, Tsuda pointed to three research papers on the 1986 Chernobyl nuclear disaster that argue that thyroid cancer was not found in a total of 47,000 children who were born after the disaster and had not been exposed to radiation, and rejects the existence of overdiagnosis in children.
Tsuda also pointed out that non-harmful cancer should have been detected in the first round of screenings, drawing attention to the fact that 51 new patients were found in the second round that began in 2014.
In regard to the results of the second round of screening, Osaka University public health professor Tomotaka Sobue, who supports the overdiagnosis theory, confesses that while it is unlikely that the cancer was caused by radiation exposure, "overdiagnosis alone cannot explain the phenomenon for now."
Cancer screenings of the same scale in other areas might help determine the main cause of the high prevalence in Fukushima children. Tsugane argues, however, that while screening is necessary in Fukushima Prefecture to confirm the effects of radiation, the same kind of screening should not be carried out in other prefectures as it will only increase the number of overdiagnosis cases.
Tsuda, on the other hand, pushes for screening in other prefectures, saying that the whole picture of thyroid cancer patients should be revealed so that the causal relationship is not blurred. In addition, he calls for the cancer registration and establishing certificates for "hibakusha" (those exposed to radiation) to confirm radiation-induced cancer patients.
Both Tsugane and Tsuda based their research on the first round of screening conducted between 2011 and 2015. About 300,000 children were screened, and thyroid cancer was detected in 113 subjects, including suspected cancer cases at the time of analysis.
Tsugane's team estimated that if all 360,000 children targeted in the cancer screening had gone through the checkups, approximately 160 patients would have been found. The team also estimated that about 5.2 children out of 360,000 children in the same age group as the Fukushima screening subjects had thyroid cancer based on calculations on a national average of thyroid cancer patients. As a result, the team reached a result of "about 30 times higher" by comparing 5.2 and 160 drawn from the estimate on Fukushima children.
Tsuda, meanwhile, focused his attention on the national average of the thyroid cancer occurrence rate in the same age group as the targets in the screening in Fukushima Prefecture, which was around three in every 1 million children per year. A total of 113 cancer patients out of 300,000 screening takers have been found in Fukushima Prefecture, which can be converted into about 90 patients in 1 million children per year over a four-year period. With those figures, Tsuda's team concluded occurrence rates of about "30 times higher."
The prefectural government's expert panel drafted the interim report based on Tsugane's calculation method.