Researchers agree that space radiation
can cause cancer. They're just not sure how.
by Karen Miller
Despite urgent warnings
from Hollywood, double-jawed aliens are probably not a spacefarer's
biggest risk. Radiation is worse. It shreds not flesh, but DNA molecules,
and that can cause a multitude of problems. One big one: it can
lead to cancer.
Oddly enough, according
to cancer specialist Dr. John Dicello of the John Hopkins University
School of Medicine, radiation "is relatively poor at inducing
cancer." Chemicals, he says, can do far more damage, as shown
by the strong tie between environmental contaminants and increased
levels of cancer.
from under Earth's atmosphere, astronauts are exposed
to space radiation
But for space travellers,
radiation is a tremendous worry. That's because astronauts are exposed
to far more radiation than we typically encounter on Earth. And
it's a different kind of radiation. Cosmic rays from deep space,
for instance, are composed of heavier particles than our bodies
are used to, and they have little trouble breaking strands of DNA.
Broken DNA, by itself,
is not necessarily cause for alarm. DNA strands break all the time.
Even a physical blow will do it. "If you hit yourself with
a hammer," notes Dicello, "that can do a lot more damage
than most radiation exposures." Because this kind of damage
occurs so frequently, the body has evolved mechanisms to handle
Sometimes, he explains,
cells with damaged DNA simply destroy themselves. Other times, they
try to repair the damage. Problems start when they do this incorrectly.
They might, for example, insert a chunk of DNA in the wrong place.
Or they might attach it to the wrong chromosome.
radiation damages DNA.
When that happens,
it's possible that the mistake will let that cell ignore constraints
designed to make cells behave. A cancer can start when altered genes
allow the cell and its descendants to multiply too freely.
So far the story sounds
simple: Radiation damages DNA. Repairs are bungled. Cancer ensues.
But it's not so simple,
says Dicello, not at all. Radiation can affect human tissue in unpredictable
ways, and the chain of events leading from radiation to cancer is
vexingly complex. "If I really understood it, I'd probably
win the Nobel Prize."
Consider the following:
Some astronauts, veterans of long space missions, have "significant
chromosome aberrations" in their blood cells. These aberrations
may be "associated
with the development of cancer," says Dicello, but they do
not, by themselves, cause cancer. For that to happen, cells with
aberrations must undergo a series of further mutations. According
to the National Cancer Institute, "the number of cell divisions
that occur during this process can be astronomically large--human
tumours often become apparent only after they have grown to a size
of 10 to 100 billion cells." Years, even decades, might pass
between the onset of the problem, the exposure to radiation, and
the appearance of a tumour.
Because of the delay,
it's very difficult to determine exactly when or why a cancer starts.
That's the bad news.
The good news - for
astronauts and for the rest of us - is that there are many places
along this slow developmental path at which an incipient cancer
can be stopped. Indeed, researchers have pinpointed some of the
genes involved and they're working on treatments targeted directly
at those genes.
of cancerous tumour. development.
Understanding how to
stop the cancer, however, doesn't necessarily tell us how
Cells often react in
unexpected ways to radiation, notes Dicello. For example, there's
a puzzling phenomenon known as adaptive response. Sometimes, when
tissue is exposed to damaging radiation, it not only repairs itself,
but also learns to repair itself better next time. How that works
is still being investigated.
damage is not always proportional to the amount of radiation experienced.
"Our research shows some unusual things," says Dicello.
Some types of chromosome aberrations are very sensitive to radiation.
"Deliver a low dose, and they take off." Other types of
aberrations require much higher doses. Researchers are still trying
to sort out which is which … and why?
make it hard to predict how human tissue will react to space radiation.
Astronauts, points out Dicello, will encounter at least two kinds
of radiation: (1) high-energy cosmic rays from distant exploding
stars and (2) less-energetic protons and photons from flares on
our own sun -- and they can be exposed to both at the same time.
While researchers know something about how cells respond to each kind
of radiation separately, some of Dicello's work suggests that exposure
to these two types of radiation mixed together could produce as-yet
specialist Dr. John Dicello
The damage could be
less than the two kinds added together - or it could be more! There
could, perhaps, be an adaptive response in which lightweight solar
protons stimulate repair processes to help reduce the effects of
the heavy cosmic ray ions. Or something totally unexpected could
There's still a lot
to learn. Dicello lists some of the questions: "How important
is adaptive response? How important is the effect of cells on each
other? How important are antioxidants? We don't yet know."
"The answers are
important to everyone," he adds. Understanding how the body
deals with damaged DNA could help doctors prevent complications
from the radiation treatments given to cancer patients. It might
help them deal with the fallout from, say, a terrorist's dirty bomb
or DNA-problems caused by environmental or chemical pollution.
believes, researchers will figure it out. And when that happens,
people on Earth will benefit at least as much as people in space…
…who can then
turn to lesser worries, like double-jawed aliens.