Five years ago, the Fukushima nuclear power plant disaster spread deadly radiation by land and by sea. In the years since, Japan and the rest of the world have learned many lessons about why the reactor failed and how to prevent another such catastrophes.

But one of the most intriguing lessons of Fukushima involves hundreds of thousands of Japanese children potentially exposed to excessive radiation.

Japanese public health officials were aware of an increased incidence of thyroid cancer in Russian children after the 1986 Chernobyl nuclear accident. So researchers screened more than 300,000 Fukushima-area children aged 18 and under for thyroid irregularities that could turn into cancer.

The scientists’ initial findings set off alarms. Tests detected some 30 times more cases of early thyroid cancer in Fukushima children than in children elsewhere in Japan who were not exposed to the reactor radiation. Many children underwent biopsies, and some even had their thyroid glands removed, a procedure that carries risks and means a child must take thyroid pills for the rest of his or her life.

But a follow-up dive into the data led many epidemiologists to doubt those early findings for several reasons. Firstly, children living closer to the accident in areas of greatest contamination had no greater rate of early cancer than those living farther away. Secondly, younger children and infants, whose thyroids are more likely to be affected than those of older children, did not show an expected higher rate of abnormal findings. Lastly, some cancers were observed less than a year after the meltdown. It generally takes years for thyroid cancer to develop after radiation exposure.

The epidemiologists’ explanation: The cancer findings were not largely attributable to the increased radiation risk, but instead occurred because tens of thousands more children were screened for thyroid cancer using sensitive ultrasound devices. Typically, those children would not have been screened and the abnormalities would not have been found.

Initially scientists had compared results from Fukushima children with all children in Japan. But when the same sensitive ultrasound was used to screen children in other parts of Japan, more thyroid cancer was found in them, too, and that 30-fold difference disappeared.

Put simply, greater screening led to more cases of early thyroid cancer diagnosed in all children. South Korean health officials report a similar finding since the government adopted widespread ultrasound screening for cancer in 1999.

Therein lies a cautionary lesson about the pros and cons of testing.

Thyroid cancer is a rare disease and usually develops slowly. Many of the ultrasound findings in the Fukushima children may never progress to full-blown cancer. An epidemic of childhood thyroid cancer deaths tied to the reactor meltdown is unlikely. Some of the children whose thyroid glands were removed might have been better off had they been placed under long-term medical observation instead.

The lesson from the Fukushima children is that intensive screening will reveal more disease, but not necessarily more disease that must be treated.

This phenomenon is called over diagnosis. Screenings find cancers that did not need to be treated because they would grow slowly, if at all. Left alone, they would probably never cause problems. But it can be difficult to distinguish between trivial abnormalities and those that ultimately cause disease.

The message to all patients -- even those an ocean away from Japan -- who are contemplating health screenings: Testing can save lives. But it can also lead to over diagnosis and even more damaging -- overtreatment. Ask your doctor. Sometimes what you do not know may not hurt you. But what you do know might.


Editorial
(Tribune Content Agency/Chicago Tribune)