Exposure to moderate levels of radiation may increase the
rates of heart disease and stroke, but the degree of risk at lower doses is
unclear, finds a large study of atomic bomb survivors published on bmj.com
Although the authors cannot be certain that this is a direct
(causal) link, their study provides the strongest evidence available to date of
an association between circulatory disease and moderate radiation exposure.
Several studies have shown that high doses of radiation to
the heart or head and neck from radiotherapy cause an excess of deaths from
heart disease or stroke in later years. But it is uncertain whether radiation
exposures at relatively low dose levels (under 1 Gy) also increase the risk.
This is an important public health issue because of the
increasing use of multiple computed tomography (CT) scans and other relatively
high dose diagnostic medical procedures.
Medical use of radiation is typically measured in milligray
(mGy). The average radiation dose from an abdominal x-ray is 1.4 mGy (0.0014
Gy), that from an abdominal CT scan is 8.0 mGy (0.008 Gy), and that from a
pelvic CT scan is 25 mGy (0.025 Gy).
To investigate this further, Dr Yukiko Shimizu and
colleagues from the Radiation Effects Research Foundation in Japan examined the
risk of heart disease and stroke in 86,611 atomic bomb survivors from Hiroshima
and Nagasaki who have been followed up for 53 years, from 1950 to 2003, as part
of the Japanese atomic bomb survivor Life Span Study.
Each member of the group had received an estimated radiation
dose from 0-4 Gy (86% received less than 0.2 Gy) at the time of the bomb. Other
risk factors for circulatory disease that could have affected the results, such
as smoking, alcohol intake, education, occupation, obesity and diabetes were
also taken into account.
The researchers found an elevated risk of both stroke and
heart disease at doses above 0.5 Gy, but the degree of risk at lower doses was
Together, this represents about 210 excess deaths from
circulatory disease associated with radiation exposure - about a third as many
as the total excess number of cancer deaths (about 625) among atomic bomb
survivors in the Life Span Study.
Adjusting for other factors made little difference to these
This study provides the strongest evidence available to date
that radiation may increase the rates of stroke and heart disease at moderate
dose levels (mainly 0.5-2 Gy), though the results below 0.5 Gy are not
statistically significant, say the authors. Further studies should provide more
precise estimates of the risk at low doses, they conclude.
This study adds to a growing body of moderate and low dose
data suggesting an association between cardiovascular disease and exposure to
low-moderate levels of radiation, as well as the well understood association at
high doses, says Mark Little from Imperial College London, in an accompanying
However, it is unclear whether the biological
mechanisms operating at high doses of radiation apply to low doses, and this
should be the focus of future research, he concludes.