An Unusual Collection of Cancers
Cancers can cluster within a community just by chance—so how
do we tell if there’s actually a dangerous cause?
By Sue Rochman
CR | Page No. 34 | Winter 2009
W
hen Kathleen Zemanek was
diagnosed with breast cancer two
years ago at age 47, she didn’t
have to look far for support. Two
of her neighbors had recently
been diagnosed with breast
cancer, as had the woman across
the street. And they weren’t alone:
There was the neighbor who had
bladder cancer, and the one with
leukemia, and at least two with
melanoma.
Zemanek wasn’t the only
person living on this bucolic San
Francisco cul-de-sac of single-family homes who had begun to
think the number of cancer cases
was unusual. Half-jokingly, they
had nicknamed themselves the
“Sixth Avenue Cancer Cluster.”
And though they didn’t talk much
about it, many wondered: Was
there something about their
neighborhood that was responsible
for all the cancer?
To date, no one who lives on
Sixth Avenue has reported any
concerns to the Environmental
Health Investigations Branch of the
California Department of Public
Health. But if they did, they’d
soon learn that theirs was one of
more than 1,000 cancer cluster
complaints made to state health
departments nationwide every
year. The vast majority do not
meet the definition of a cluster
and are never investigated. But
even when the health department
believes a situation looks
suspicious, it’s far from easy to
find a cause. In fact, more often
than not, the investigators who
study a cluster, and those affected
by it, are left with more questions
than answers.
CLUSTER BUSTERS
Defining a cancer cluster is easy:
It’s an unusually high number
of cancers that occur during a
specific period of time among
people who live in the same
geographic area, share the same
workplace, or have the same
occupation. But determining
whether a group of cancers is
not a random event but an actual
cancer cluster—and if so, what
might have caused it—is one of
the more difficult tasks public
health officials face.
“Clusters are mathematically
going to occur all of the time,”
explains Alfred I. Neugut, an
epidemiologist at Columbia
University in New York City. “If you
flip a coin, you might get three
heads in a row, or five. That’s
clustering.” And this happens with
cancer cases all the time. Imagine
you’d expect to see 110 cases of a
particular type of cancer in every
100,000 women, he continues.
“Not every group of 100,000
women will have 110 cases. Some
might have 90 and others might
have 150. … And it has nothing
to do with etiology or anything
sinister under the surface.”
Except when it does.
“The overwhelming majority
of cancer clusters are statistical
fluctuations that don’t
indicate anything at all,” says
epidemiologist Wendel Brunner,
the director of public health for
the health services department of
Contra Costa County, in California.
