Cancer Prevention
According to the American Cancer Society (ACS), more
than a half million Americans will die of cancer in 2004-that is more than 1,500
people a day. One of every four deaths in America is from cancer. About 1.4
million new cases of cancer will be diagnosed in 2004. This estimate does not
include diagnoses of in situ (preinvasive) cancer (except for urinary bladder
cancer) or the approximately 1 million cases of nonmelanoma skin cancer that will be diagnosed
this year.
The National Cancer Institute (NCI) estimates that about
9.6 million Americans with a history of cancer were alive in January 2000.
Although some of these individuals were considered to be cured or cancer free,
others continued to live with the disease and may have been receiving treatment.
Effective strategies exist for reducing the number of both new cases of
cancer and deaths caused by cancer. These include decreasing the prevalence of behavioral and
environmental factors that increase people's cancer risk and ensuring that
evidence-based screening tests and treatment services are available and
accessible. Resources that can be devoted to such strategies are limited, and
must be allocated wisely. Wise resource allocation, in turn, depends partly on
the availability of complete, timely, and high-quality cancer data.
Much of the promise for cancer prevention comes from observational
epidemiologic studies that show associations between modifiable life style
factors or environmental exposures and specific cancers. Evidence is now
emerging from randomized controlled trials
designed to test whether interventions suggested by the epidemiologic studies,
as well as leads based on laboratory research, result in reduced cancer
incidence and mortality.
The most consistent finding, over decades of research,
is the strong association between tobacco use and cancers of many sites.
Hundreds of epidemiologic studies have confirmed this association. Further
support comes from the fact that lung cancer death rates in the United States have mirrored
smoking patterns, with increases in smoking followed by dramatic increases in
lung cancer death rates and, more recently, decreases in smoking followed by
decreases in lung cancer death rates in men.
Additional examples of modifiable cancer risk factors
include alcohol consumption (associated with increased risk of oral, esophageal,
breast, and other cancers), physical inactivity (associated with increased risk
of colon,
breast, and possibly other cancers), and being overweight (associated with colon,
breast, endometrial, and possibly other cancers). Based on epidemiologic
evidence, it is now thought that avoiding excessive alcohol consumption, being
physically active, and maintaining recommended body weight, may all contribute
to reductions in risk of certain cancers; however, compared with tobacco
exposure, the magnitude of effect is modest or small and the strength of
evidence is often weaker. Other lifestyle and environmental factors known to
affect cancer risk (either beneficially or detrimentally) include certain sexual
and reproductive practices, the use of exogenous estrogens, exposure to ionizing
radiation and ultraviolet radiation, certain occupational and chemical
exposures, and infectious agents.
Food and nutrient intake have been examined in relation
to many types of cancer. Fruit and vegetable consumption have generally been
found in epidemiologic studies to be associated with reduced risk for a number
of different cancers. Contrary to expectation, randomized trials found no
benefit of beta-carotene supplementation in reducing lung cancer incidence and
mortality; risk of lung cancer was statistically significantly increased in
smokers in the beta-carotene arms of 2 of the trials. Similarly, randomized
controlled trials have found no reduction in risk of subsequent adenomatous
polyps of the colon in individuals who have had polyps previously resected
taking dietary fiber supplements compared with those receiving much lower
amounts of supplemental wheat bran fiber. On the other hand, there is evidence
from at least 1 randomized controlled trial that calcium supplementation does
modestly reduce risk of adenoma recurrence. Consumption of red meat and
inadequate folic acid intake have also been
associated with increased risk of colon cancer. A large randomized trial is currently underway to investigate
whether men taking daily selenium or vitamin E or both experience a
reduced incidence of prostate cancer in comparison to men taking placebo pills. A
meta-analysis of 19 randomized trials suggests that vitamin E supplements do not
decrease the risk of all-cause mortality.
Daily use of tamoxifen, a selective estrogen receptor
modulator, for up to 5 years, has been demonstrated to reduce the risk of
developing breast cancer in high-risk women by about 50%. Cis-retinoic acid also
has been shown to reduce risk of second primary tumors among patients with
primary cancers of the head and neck. Finasteride, an alpha-reductase inhibitor,
has been shown to lower the risk of prostate cancer. Other examples of drugs that show promise for
chemoprevention
include COX-2 inhibitors
(which inhibit the cyclooxygenase enzymes involved in the synthesis
of proinflammatory prostaglandins).
Considerable research effort is now devoted to the
development of vaccines to prevent infection by oncogenic agents, and to
potential venues for gene therapy for individuals with genetic mutations or polymorphisms that put them at high
risk of cancer. Meanwhile, genetic testing for high-risk
individuals, with enhanced surveillance or prophylactic surgery for those who test positive, is
already available for certain types of cancer, including breast and colon
cancers.
Screening for colon cancer through fecal occult blood
testing has been demonstrated to reduce both colon cancer incidence and
mortality, presumably through the detection and removal of precancerous polyps.
Similarly, cervical cytology testing (using the Pap smear) leads to the
identification and excision of precancerous lesions. Over time, such testing has
been followed by a dramatic reduction of cervical cancer incidence and mortality.
Sources: Centers for
Disease Control and the
National Cancer Institute
Last Editorial Review: 5/4/2005
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