We could infer that exposure doses for residents were higher than the official report or the WHO's dose estimation, because the number of thyroid cancer cases grew faster than predicted in the WHO's health assessment report.
The Japanese government has made its first admission that a worker at the Fukushima nuclear plant developed cancer as a following decontamination work after the 2011 disaster.
The man worked at the damaged plant for over a year, during which he was exposed to 19.8 millisieverts of radiation, four times the Japanese exposure limit. He is suffering from leukemia.
The former Fukushima manager Masao Yoshida also contracted cancer of the oesophagus after the disaster and died in 2013 - but the owner and operator of the nuclear plant, Tepco, refused to accept responsibility, insisting that the cancer developed too quickly.
Three other Fukushima workers have also contracted cancer but have yet to have their cases assessed.
The Fukushima nudear disaster followed the tsunami of 11 March 2011. Three out of four reactors on the site melted down, clouds of deadly radiation were released following a hydrogen explosion, and the nuclear fuel appears to have melted through the steel reactor vessels and sunk into, or through, the concrete foundations.
The tip of an iceberg
But that single 'official' cancer case is just the beginning. New scientific research indicates that hundreds more cancers have been and will be contracted in the local population.
A 30-fold excess of thyroid cancer has been detected among over 400,000 young people below the age of 18 from the Fukushima area.
According to the scientists, "The highest incidence rate ratio, using a latency period of 4 years, was observed in the central middle district of the prefecture compared with the Japanese annual incidence."
In a first screening for thyroid cancer among 298,577 young people four years after the disaster, thyroid cancer occurred 50 times more among those in the most heavily irradiated areas, than in the general population, at a rate of 605 per million examinees.
In a second screening round of 106,068 young people conducted in April 2014 in less irradiated parts of the prefecture, the cancer was 12 times more common than for the main population.
Thyroid cancer is commonly developed as a result of acute exposure to radioactive iodine 131, a product of nuclear fission. Because iodine concentrates in the thyroid gland, thyroid damage including cancer is a characteristic marker of exposure to nuclear fallout.
Exposure to iodine-131 presents a high risk in the immediate aftermath of a nuclear accident owing to its short half life of 8 days, making it intensely radioactive. It is estimated to have made up about 9.1% of the radioactive material released at Fukushima.
There's many more cases on the way!
The paper's authors note that the incidence of thyroid cancer is high by comparison with the Chernobyl nuclear disaster in 1986 at the same time following exposure - and warn that many more cases are likely to emerge:
"In conclusion, among those ages 18 years and younger in 2011 in Fukushima Prefecture, approximately 30-fold excesses in external comparisons and variability in internal comparisons on thyroid cancer detection were observed in Fukushima Prefecture within as few as 4 years after the Fukushima power plant accident. The result was unlikely to be fully explained by the screening effect.
"In Chernobyl, excesses of thyroid cancer became more remarkable 4 or 5 years after the accident in Belarus and Ukraine, so the observed excess alerts us to prepare for more potential cases within a few years."
Scientific studies of Chernobyl victims have also found that the risk of developing thyroid cancer has a long, fat tail - in other words, there is no significant fall in risk over time among people exposed to iodine-131.
According the the US's National Cancer Institute, summarising the findings in 2011, "The researchers found no evidence, during the study time period, to indicate that the increased cancer risk to those who lived in the area at the time of the accident is decreasing over time.
"However, a separate, previous analysis of atomic bomb survivors and medically irradiated individuals found cancer risk began to decline about 30 years after exposure, but was still elevated 40 years later. The researchers believe that continued follow-up of the participants in the current study will be necessary to determine when an eventual decline in risk is likely to occur."
Did WHO underestimate the Fukushima radiation release?
The authors of the Fukushima study also suggest that the amount of radiation released may, in fact, have been more that the World Health Organisation's and other official estimates:
"Furthermore, we could infer a possibility that exposure doses for residents were higher than the official report or the dose estimation by the World Health Organization, because the number of thyroid cancer cases grew faster than predicted in the World Health Organization's health assessment report."
Another consideration - which the authors do not enter into - is the effect of the other radioactive species emitted in the accident including 17.5% Caesium-137 and 38.5% Caesium 134. These longer lived beta-emitters (30 years and two years respectively) present a major long term hazard as the element is closely related to potassium and readily absorbed into biomass and food crops.
Yet another radiation hazard arises from long lived alpha emitters like plutonium 239 (half life 24,100 years) which is hard to detect. Even tiny nano-scale specks of inhaled plutonium entering the lungs and lymphatic system can cause cancer decades after the event by continuously 'burning' surrounding tissues and cells.
The paper: 'Thyroid cancer detection by ultrasound among residents aged 18 years and under in Fukushima, Japan: 2011 to 2014' is published in Epidemiology.
Oliver Tickell edits The Ecologist.