[ RadSafe ] BBC Reports "Nuclear workers' cancer risk rise

[Franta, Jaroslav]

NUCLEONICS WEEK JULY 7, 2005
Nuclear workers study supports linear no-threshold risk model

The largest epidemiological study of low-dose radiation
risk so far supports existing radiation protection standards
that assume even tiny doses can cause cancer but adjust risk
estimates downward for low doses, the study's authors said last week.
The "International Collaborative Study" of cancer risk for
407,391 nuclear industry workers in 15 countries showed a
slight but statistically significant excess risk of cancer even
at the low doses and dose rates typically received by the
workers, the scientists said.
They estimated that 1%-2% of the cancer deaths recorded
among the population studied were attributable to radiation exposure.
The study is the first to provide solid evidence of a linear
no-threshold (LNT) model for low-dose radiation risk. That
model, based on the high doses and dose rates sustained by
Japanese atomic bomb survivors, holds that the risk should
be extrapolated linearly down to zero dose. The LNT model
is contested by some scientists worldwide who contend that
doses below 100 milliSievert (mSv) or even 200 mSv are
harmless and may even be beneficial. But there are also
those who argue that low doses and low dose rates are actually
more dangerous than higher ones.
The results of the new study were published June 29 in
the British Medical Journal (
http://bmj.bmjjournals.com/onlinefirst_date.shtml ). 
The team of scientists was led by Elisabeth Cardis, head of the radiation
group at the
International Agency for Research on Cancer (IARC)
( http://www.iarc.fr/ENG/Units/RCAa1.html ) in Lyon, France.
On June 29, the U.S. National Academies' National
Research Council published a study on low-dose health
effects that also supported the LNT model (NW, 30 June, 12).
An earlier IARC collaborative study with a much smaller
cohort covering only the U.S., U.K. and Canada, published
in 1994, had not found a statistically significant excess risk
for all cancers other than leukemia (NW, 27 Oct. '94, 1). 
For leukemia, that study found an excess relative risk per Sievert
of 2.2, lower than the 3.7/Sv extrapolated linearly from the
survivors of atomic bombings at Hiroshima and Nagasaki,
the main source of estimates of radiation risk.
At the time, Cardis had noted that the follow-up study
was needed to draw more solid conclusions, since most
members of the earlier cohort were still alive and solid cancers
have a much longer latency period than leukemia.
The new study, which has been eagerly awaited but took
over a year to publish, found an excess relative risk for cancers
other than leukemia of 0.97 per Sv, with a confidence
interval of 0.14 to 1.97 (i.e., excluding an absence of risk).
The excess relative risk for leukemia (excluding chronic lymphocytic
leukemia, CLL, which is considered not radiation-related)
was 1.93 per Sv, with a confidence level of 0 to 8.47.
Those estimates indicate that for a dose of 100
milliSievert (mSv), currently the five-year limit for occupational
radiation exposure in international rad protection
standards, the risk of dying from solid cancer is 9.7% higher
than that for natural cancer mortality, and the risk of dying of leukemia is
19% higher.
The latest study covered workers in the nuclear industry,
except uranium mining, between 1944 and 2000 in the
three original countries plus France, Japan, South Korea,
Sweden, Belgium and seven other European countries. 
The overall average cumulative recorded dose was 19.4 mSv, but
the distribution of recorded dose was greatly skewed. Ninety
percent of workers in the cohort received cumulative doses
of under 50 mSv, less than 5% received doses of 100 mSv
over their career, and less than 0.1% received cumulative
doses above 500 mSv. Most of the higher doses were
received in the early years of the nuclear industry "when
protection standards were less stringent than today," the scientists wrote.
A total of 24,158 people in the cohort were known to
have died during the study period; 6,519 of them died from
cancers other than leukemia and 196 from leukemia excluding CLL.
Based on their findings, the scientists estimated that
about 1%-2% of the deaths from cancer among workers in
the larger cohort "may be attributable to radiation," the scientists wrote.
The central risk estimates for all cancers (excluding
leukemia) are between two and three times higher than the
linear extrapolation from atomic bomb survivors, the scientists
wrote, but because the confidence intervals are wide,
their findings are "statistically compatible with the current
bases for radiological protection."
Compatible risk estimate
As for leukemia, the scientists said, the central risk estimate
from the new study-which is similar to estimates
found in earlier large-scale nuclear worker studies-is compatible
with the current approach of radiation protection,
i.e., a linear dose-response model with no threshold but
dividing risk estimates by two to allow for assumed reduced
effect at low dose rates. However, they said "the confidence
interval is wide" and the findings are also compatible with
no reduction at low doses and dose rates, as well as with
greater reduction of risk at low doses. In fact, they said, their
central risk estimate for leukemia is halfway between estimates
obtained by fitting a linear dose-response model and a
linear-quadratic model to data on men exposed to the atomic bomb at age
20-60.
In an interview, Cardis noted that compared to the previous
collaborative study of three countries' workers, the new
study includes many more individuals with lower doses, primarily
because there are more recent data-the early doses
were the highest-and the cohort is much larger. A graph in
the paper shows more than 250,000 of the workers at doses
under 10 mSv, most of the others under 50 mSv, and only
isolated individuals with doses up to 1.6 Sv.
Cardis said the research team had excluded all individuals
judged to have received 10% or more of their dose from
internal contamination or neutrons, which eliminated some
of the higher doses. The team also analyzed risk by time
periods to see the effect of dose levels. Cardis acknowledged
that "the statistical significance of the results is driven by
the higher-dose workers." If doses higher than 200 milliGray
(200 mSv) were eliminated from the calculations, she said,
the risk estimate "was similar but no longer statistically significant."
Risk comparisons based on time period and excluding
the higher-dose workers will be included in a second, more
detailed publication of the study results, she said.
The fact that the estimated risk from lung cancer was
particularly high among the cohort suggests that smoking
may have been a confounding risk factor in the study.
Cardis acknowledges that the researchers did not have
access to clear enough data about the workers' smoking
habits and that smoking may well be responsible for part of the cancer risk.
However, she said analysis had shown a "much lower"
risk of smoking-related cancers excluding lung cancer, and
said all the risk estimates-for mortality from all groups of
cancers, both related and unrelated to smoking-were consistently
higher than the risk estimate for solid cancers from
the atomic bomb analysis. That, she said, suggests that
"although smoking is playing a role (in the deaths), it is not the only
factor."
Cardis said that as a next step, IARC is starting case-control
studies of lung cancer and of leukemia, nested within
cohorts of workers who have been employed in uranium
and plutonium processing facilities.
Those studies, she said, "will allow us to better quantify
the risk of these malignancies related to both internal and
external exposures and to take into account the possible
confounding effect of smoking and of other occupational carcinogens."
She said IARC would "also like to extend the follow-up of
the cohorts included in the 15-country study."
-Ann MacLachlan, Paris
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