Ultrasound shows
thyroid cancer spike after Fukushima
October 14, 2015 --
Just as many had feared, the disaster at the Fukushima Daiichi nuclear power
plant in March 2011 has sparked a sharp increase in thyroid cancer among
children and adolescents in the area, according to a study published online in Epidemiology.
And more cases may be lurking.
One
particularly hard-hit district in the Fukushima Prefecture saw thyroid cancer
rates that were 50 times higher than the national average in Japan among those
18 and younger, reported researchers who participated in a thyroid ultrasound
screening program in the three years after the earthquake. Cancer rates
throughout the prefecture are 30 times higher than in Japan as a whole.
"The
result was unlikely to be fully explained by the screening effect," wrote
the research team led by Dr. Toshihide Tsuda, PhD, of Okayama University.
"In Chernobyl, excesses of thyroid cancer became more remarkable four or
five years after the accident in Belarus and Ukraine, so the observed excess
alerts us to prepare for more potential cases within a few years."
Radiation
exposure
Following
the meltdown of three Fukushima nuclear reactors, approximately 900
petabecquerel of radiation was released into the atmosphere, the radiological
equivalent of one-sixth of the 5,200 petabecquerel released by the Chernobyl
disaster. Based on its preliminary dose estimation in 2012, the World Health
Organization (WHO) predicted there would be an increase in thyroid cancers
among children who had been exposed to the radiation.
WHO
estimated that those in the most affected areas of the prefecture received
thyroid-equivalent doses of 100-200 mSv, while those in the rest of the
prefecture received 10-100 mSv via inhalation, ingestion, and external exposure
from fallout deposits on the ground, according to the researchers (Epidemiology,
October 5).
Nearly
four years after the accident, the group sought to determine accurate and
quantitative estimates from the Fukushima experience to plan for the
population's future health needs.
Thyroid
ultrasound screening
Under a
screening program planned and conducted by the government of the Fukushima
Prefecture, all residents 18 years or younger received thyroid ultrasound
screening sometime during the 2011-2013 fiscal years. Screening was performed
in three areas: in 2011 in the area nearest the disaster, mostly within 50 km
of the Fukushima plant; in 2012 in a middle area of the prefecture; and in 2013
in the least-contaminated area.
A
second round of screening, which will also include residents born in the
prefecture between April 2, 2011, and April 1, 2012, began in April 2014 and is
expected to be completed in March 2016.
Those
with positive ultrasound findings on screening received a secondary exam, followed
by fine needle aspiration (FNA) biopsy if necessary. Patients with detected
cancer cells were followed and operated on at an appropriate time, according to
the researchers.
Of the
367,687 residents 18 years or younger in the prefecture in 2011, 298,577 (81%)
had received the first round of screening by the end of December 2014. There
were 2,251 positive thyroid ultrasound cases by 2014 and 2,067 received a
secondary exam.
From
these, FNA indicated the presence of 110 thyroid cancers, 87 of which had been
operated on by the end of 2014. On histological examination, 86 were confirmed
to be malignant, a number that included 83 papillary carcinomas and three
poorly differentiated carcinomas.
The
researchers compared the prevalence of thyroid cancer for each area by
calculating a prevalence odds ratio in comparison with the least-contaminated
district of the least-contaminated area of the prefecture. For a comparison
with subjects outside of the prefecture, the team calculated incidence rate
ratios in comparison with annual incidence rates nationwide in Japan, taking
into account the prevalence as well as the latent duration (four years) of the
disease.
Thyroid cancer prevalence
|
Area
|
Prevalence of thyroid cancer per 1 million people
|
Prevalence odds ratio compared to least-contaminated district
|
Incidence odds ratio compared to Japan national rates
|
Nearest to Fukushima
|
359
|
1.5
|
30
|
Middle -- not evacuated
|
402
|
1.7
|
33
|
Least contaminated
|
332
|
N/A
|
28
|
The
researchers noted that the highest incidence odds ratio (50; 95% confidence
interval: 25-90) in comparison with the mean Japanese annual incidence of
thyroid cancer was found in the middle area's central middle district -- 50 to
60 km west of the power plant -- where residents were not evacuated. That
district had a prevalence rate of 605/1,000,000 (95% confidence interval:
302-1,082) and a prevalence odds ratio in comparison with the
least-contaminated district of 2.6 (95% confidence interval: 0.99-7.0).
The
finding that southernmost districts within the middle and the
least-contaminated areas of the prefecture had higher incidence rates than the
northernmost districts was consistent with the flow of indium-131 being
primarily in a southern direction from the Fukushima release.
2nd
round of screening
In the
second screening round that began in 2014, 106,068 (49%) of 218,397 total
subjects have been screened so far. Of the 71% of exams with a decision as to
whether the secondary exam was necessary or not, there were 611 positive
studies.
The
secondary exam has been performed on 377 subjects (62% of the positive
studies), and 262 received a final diagnosis on the secondary exam. FNA was
performed on 22 subjects (8%) and detected eight new thyroid cancer cases by
cytology as of December 31, 2014. All eight had negative ultrasound findings in
their first round of screening.
New
data released in May 2015 added two additional thyroid cancer cases from the
first round of screening and seven more cases from the second round (for a
total of 15). As a result, the incidence ratio so far from the second round of
screening is 13.7 (95% confidence interval: 7.7-23).
The
authors concluded that within as few as four years after the disaster, there
has been an approximately 30-fold increase in thyroid cancer among children and
adolescents in the Fukushima Prefecture. As the number of thyroid cancer cases
has increased faster than predicted by the WHO, it's possible that the
organization's previous exposure estimates for residents were too low,
according to the group.
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