Likewise, excessive exposure to ultraviolet light (UV), a form
of radiation, is a risk factor for skin cancer, particularly the
most lethal form, melanoma. New standards for sunscreen,
increased sun protection, and decreased UV exposure,
including avoiding the use of tanning beds, should greatly
reduce melanoma and other skin cancers.
Exposure to the naturally occurring radioactive gas, radon,
causes between 15,000 and 22,000 lung cancer deaths a
year, making it the second leading cause of lung cancer after
smoking16. This discovery has led to policies for reducing
exposure through home and business inspections, and the
containment or elimination of the source when possible.
Increased awareness along with these mitigation strategies
should greatly reduce the incidence of lung cancer caused
by these exposures.
The role of environmental and workplace exposures in
cancer and other health outcomes has been an important
area of epidemiologic and toxicologic research. Agents such
as asbestos and the related volcanic rock, erionite, cause an
aggressive form of cancer, called mesothelioma, that is
difficult to treat. Chemicals like arsenic, aflatoxin, and
pesticides, particularly dichlorodiphenyltrichloroethane
(DDT), are also associated with an increased risk of a variety
of cancers17. Our knowledge of their roles in causing cancer
has paved the way for important preventive interventions
and public policy. Our understanding of how overall
exposure, length of exposure, and exposures to multiple
toxins contributes to the formation of cancer is still
incomplete and requires further study.
Health Behaviors in
Cancer Risk and Prevention
In parallel with the many advances in cancer research at the cellular and molecular
level, there has been tremendous progress in identifying health behaviors that can
affect cancer risk.
A major challenge has been that risk factors for the 200 different types of cancer
can differ substantially. For example, some cancers are predominantly dependent on
hormonal factors, such as breast, endometrial, or prostate cancer, while others are
strongly influenced by dietary components, such as tumors of the gastrointestinal
tract. Tobacco smoking is a nearly universal villain, increasing the risk of developing
cancer at no less than 18 different tumor sites and accounting for 30% of all cancer
cases and deaths in the U.S.
Based on the epidemiologic research to date, it is conservatively estimated that 50%
of cancers would be preventable by health behavioral changes. This number gives
us hope that cancer prevention measures can make a substantial impact on the
lives of millions of Americans, while at the same time save billions in health care
A major achievement in cancer prevention has been the 2007 World Cancer
Research Fund report, entitled “Food, Nutrition, Physical Activity and the Prevention
of Cancer – A Global Perspective”, summarizing four decades of epidemiologic
research on this topic. The 7,000 international research studies draw a clear picture
of where dietary changes can be successful and where more work is needed.
Avoiding red meat, particularly well-cooked red meat, reduces colon cancer risk,
while fruits and vegetables appear generally beneficial with strong risk reductions
for lung and gastric cancer.
If diet can have such a large effect on cancer risk, the question remains whether we
will be able to prevent cancer by taking vitamin pills. Large-scale prevention trials
enrolled thousands of participants to determine if beta-carotene could prevent lung
cancer. The results showed not only no benefit, but actual harm. Similar trials are
underway to determine if other molecules that are present in fruits and vegetables
can prevent cancer.
Another important area of research is the role of vitamin D in colon cancer
prevention. Studies have reported that higher vitamin D levels in the blood
correspond to reduced risks and better survival; however, vitamin D is also produced
in the skin and is linked to physical activity, which complicates analysis of its
efficacy. Several NIH-funded trials using vitamin D supplements are underway, and
cancer researchers are anticipating the official reports of these studies.
Yet another potential anti-cancer pill comes from a different direction, the non-steroidal anti-inflammatory drugs (NSAIDs). Inflammation fosters the growth of
tumors in both humans and animals, and epidemiologic studies in the 1980s
reported decreased risks of colorectal cancer among regular aspirin users. In 2003,
the first clinical trials reported success in the reduction of colorectal polyps among
those taking NSAIDs. Recently, analyses of several large clinical trials showed that
more than 5 years of a small dose of aspirin resulted in a 40% reduction in
colorectal cancer mortality, but also a significant reduction in the overall cancer
We do not have a “wonder pill” to prevent cancer and, because of the complexity of
cancer, it is unlikely that any one chemopreventive agent is going to offer the
desired results. The benefits among women are not clear, and aspirin and other
drugs can have side effects. Today’s research focuses on identifying genetic factors
that predispose individuals to these and other side effects – with the hope that we
will be able to harness the potential of NSAIDs and other chemopreventive agents
and offer personalized cancer prevention to high-risk populations (see Molecularly
Based Prevention, p. 65).
We now know that what individuals do on a daily basis can help reduce their cancer
risk. Our tools for measuring genetic risk factors, as well as lifestyle and health
behaviors, have improved significantly, providing great potential for fruitful research
that will close the numerous gaps in our knowledge. Cancer prevention research is a
critical component of our goal to conquer cancer, and there is much optimism that
success in this area will reduce the burden of cancer to society.