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Guilt by Association? Some Cancer Epidemiology Favorites

Nancy Volkers

You may have seen the titles of some published cancer research articles and wondered: Why? Why study the relationship between owning a pet bird and lung cancer? Why study cancer risk in Lapps who herd reindeer?

Actually, valuable information can be obtained from such studies, though they may sound a bit frivolous at first blush. Below we have compiled results from some of these studies that have been published over the last decade or so, with some background on why anyone would study this stuff in the first place. The citations for the articles used for this story are available as an online supplement at http://jncicancerspectrum.oupjournals.org.

Where’s the Beef?

Butchers and slaughterhouse workers don’t have the most glamorous professions; on top of that, they may have an increased risk of some cancers. A recent study found that Swedish butchers and meat workers had a 60% increased risk of stomach cancer and oral/pharyngeal cancer and a 40% increased risk of lung cancer compared with their peers who do not regularly come into contact with animals. The stomach cancer risk was highest among urban butchers. A case–control study in New Zealand found a fivefold increased risk of adult acute leukemia among slaughterhouse workers with more than 2 years of direct animal contact. Butchers who worked in slaughterhouses or on farms had increased risks of leukemia, but retail and wholesale butchers and meatpackers did not.

Finally, a U.S. study done in 1998 found increased risks of hematologic and lymphatic tumors in slaughterhouse workers and in butchers who killed animals. Supermarket workers who wrapped meat had nearly four times the risk of such tumors, and supermarket meatcutters had 18 times the risk of multiple myeloma.

These increased risks, say researchers, could result from lifestyle factors or to industry-specific exposures, such as viruses, pesticides, and other chemicals.

Up, Up and Away

GoThe friendly skies may not be all that friendly if you’re up there long enough: Exposure to cosmic radiation could increase your risk of cancer. At least some research has supported this idea, finding that female flight attendants have increased risks of breast cancer, melanoma, and sarcomas. An Icelandic study that used 1955–1997 data from female flight attendants found an increased overall risk of cancer that was even higher in those hired in 1971 or later.



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Not so friendly skies? Some studies have found an increased risk of some cancers among flight attendants.

 
Although less data are available, men also seem to be at risk. A study that included male flight attendants found no increased risk of cancer, but a 2000 meta-analysis found that male pilots had increased risks of prostate cancer and of death from melanoma. However, a recent review of mortality statistics in both male and female flight personnel found that both had lower risks of cancer death.

Getting a Leg Up

Hip replacement surgery introduces foreign materials into the body, which researchers warn may eventually lead to increased cancer risk, particularly of the bone or connective tissue near the surgery site. However, anyone who worries that getting a hip replaced significantly increases their cancer risk probably doesn’t have a leg to stand on.

Although a 1996 study found a 2% to 8% overall increased risk of cancer in people with hip prostheses, two more recent studies gave some contradictory results. One of the studies did find slightly increased risks specifically for prostate cancer, melanoma, and multiple myeloma, but no increased risks of bone or connective tissue cancers. The other study found that hip replacement patients had a 10% overall decreased risk of cancer, including a 30% decreased risk of lung cancer and a 23% decreased risk of stomach cancer.

No Marriage Penalty?

Some marry for love, and some for money ... but few of us marry (or divorce) to alter our cancer risk. Still, researchers have analyzed quite a bit of data on female marital status and how it affects cancer risk and mortality. They have found that, although unmarried women have higher risks of hematologic cancers, brain cancer, ovarian cancer, and uterine cancer than married women, they have lower risks of thyroid and cervical cancer. Widowed women have no increased cancer risk, but the results for divorced women are mixed: Widows have increased risks for lung and cervical cancers but lower risks of thyroid, endometrial, colorectal, and hematologic cancers.

Among other hypotheses, the social supports of marriage are believed to reduce cancer mortality risk. Compared with married women, unmarried women have a 26% increased risk of dying from cancer. Widows have twice the risk of death from colorectal cancer, and divorcees have increased risks of death from breast, lung, and cervical cancer.

The Night Shift

Some people claim to enjoy working nights, and the pay is often better. It turns out, however, that there may be a down side. Exposure to light at night could increase the risk of breast cancer by suppressing the production of melatonin, which would increase estrogen levels.

A case–control study published in 2001 found that working the night shift was associated with a 60% increased risk of breast cancer. A larger case–control study involving more than 7,000 Danish women concluded that women who worked at night for at least 6 months had a 50% increased risk of breast cancer. In both studies, risk tended to increase with more years of night-shift work.

Finally, data from the Nurses’ Health Study—which includes more than 78,000 people—show that those who worked night shifts for more than 30 years had a 36% increased risk of breast cancer. The study also found near significant increased risks for nurses working less than 30 years.

The Truth About Cats and Dogs

GoPet birds have an impressive publications list when it comes to epidemiologic studies of cancer risk. Cats and dogs, on the other hand often get the short end of the leash. But there is some data on Fluffy and Fido, either as "sentinels" of environmental exposures or as possible carriers of viruses that could increase cancer risk in the humans with whom they live.



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A study published earlier this year found that pet cats exposed to environmental tobacco smoke have an increased risk of lymphoma compared with cats that lived in smoke-free homes.

 
A 2002 study highlighted another reason to give up cigarettes: Pet cats who were exposed to environmental tobacco smoke had more than twice the risk of lymphoma compared with cats that lived in smoke-free homes. Those with 5 or more years of exposure had 3.2 times the risk.

Dogs, it seems, are safer from the effects of smoking: a 1992 study found no increased risk of lung cancer in dogs exposed to passive smoke in the home. A weak increase in risk was found for dogs with short or medium-length noses, the results weren’t statistically significant, so the jury is still out.

Finally, worries about pet ownership and the risk of some childhood cancers seem to be unfounded. A case–control study of more than 2,300 children in the United States and Canada found no relationship between childhood leukemia and living with a dog or cat. Sick pets were no more likely than healthy ones to contribute to leukemia risk.



             
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