| Cancer prevention is defined as active | | | | have been validated in an observational |
| measures to decrease the incidence of | | | | (or occasionally a prospective |
| cancer. This can be accomplished by | | | | interventional) trial in humans. |
| avoiding carcinogens or altering their | | | | The case of beta-carotene provides an |
| metabolism, pursuing a lifestyle or diet | | | | example of the necessity of randomized |
| that modifies cancer-causing factors and | | | | clinical trials. Epidemiologists |
| or medical intervention | | | | studying both diet and serum levels |
| (chemoprevention, treatment of | | | | observed that high levels of |
| pre-malignant lesions). | | | | beta-carotene, a precursor to vitamin A, |
| Much of the promise for cancer | | | | were associated with a protective |
| prevention comes from observational | | | | effect, reducing the risk of cancer. |
| epidemiologic studies that show | | | | This effect was particularly strong in |
| associations between modifiable life | | | | lung cancer. This hypothesis led to a |
| style factors or environmental exposures | | | | series of large randomized trials |
| and specific cancers. Evidence is now | | | | conducted in both Finland and the United |
| emerging from randomized controlled | | | | States (CARET study) during the 1980s |
| trials designed to test whether | | | | and 1990s. This study provided about |
| interventions suggested by the | | | | 80,000 smokers or former smokers with |
| epidemiologic studies, as well as leads | | | | daily supplements of beta-carotene or |
| based on laboratory research, actually | | | | placebos. Contrary to expectation, these |
| result in reduced cancer incidence and | | | | tests found no benefit of beta-carotene |
| mortality. | | | | supplementation in reducing lung cancer |
| Examples of modifiable cancer risk | | | | incidence and mortality. In fact, the |
| include alcohol consumption (associated | | | | risk of lung cancer was slightly, but |
| with increased risk of oral, esophageal, | | | | not significantly, increased by |
| breast, and other cancers), smoking | | | | beta-carotene, leading to an early |
| (although 20% of women with lung cancer | | | | termination of the study. [10] |
| have never smoked, versus 10% of men | | | | However, Randomized Clinical Trials |
| [1]), physical inactivity (associated | | | | (RCTs) also have drawbacks in cancer |
| with increased risk of colon, breast, | | | | prevention, particularly in |
| and possibly other cancers), and being | | | | micronutrient deficiencies, which are |
| overweight (associated with colon, | | | | thought by some to be a major |
| breast, endometrial, and possibly other | | | | contributor to cancer. RCTs involve huge |
| cancers). Based on epidemiologic | | | | numbers of people, take many years to |
| evidence, it is now thought that | | | | complete, and are therefore extremely |
| avoiding excessive alcohol consumption, | | | | expensive and complicated, and therefore |
| being physically active, and maintaining | | | | few are done. In addition, these |
| recommended body weight may all | | | | randomized clinical trials usually test |
| contribute to reductions in risk of | | | | only a single dose. An alternative, |
| certain cancers; however, compared with | | | | which is likely to be more useful, is to |
| tobacco exposure, the magnitude of | | | | do shorter intervention trials focusing |
| effect is modest or small and the | | | | on other endpoints related to cancer, |
| strength of evidence is often weaker. | | | | such as DNA damage. These trials can |
| Other lifestyle and environmental | | | | test a variety of doses on fewer people |
| factors known to affect cancer risk | | | | to determine what level of micronutrient |
| (either beneficially or detrimentally) | | | | intake (or, better, micronutrient |
| include certain sexual and reproductive | | | | concentration in blood) keeps DNA damage |
| practices [citation needed], the use of | | | | to a minimum. |
| exogenous hormones, exposure to ionizing | | | | Other chemoprevention agents |
| radiation and ultraviolet radiation, | | | | Daily use of tamoxifen, a selective |
| certain occupational and chemical | | | | estrogen receptor modulator, typically |
| exposures, and infectious agents. | | | | for 5 years, has been demonstrated to |
| Diet and cancer | | | | reduce the risk of developing breast |
| The consensus on diet and cancer is that | | | | cancer in high-risk women by about 50%. |
| obesity increases the risk of developing | | | | Cis-retinoic acid also has been shown to |
| cancer. Particular dietary practices | | | | reduce risk of second primary tumors |
| often explain differences in cancer | | | | among patients with primary head and |
| incidence in different countries (e.g. | | | | neck cancer. Finasteride, a 5-alpha |
| gastric cancer is more common in Japan, | | | | reductase inhibitor, has been shown to |
| while colon cancer is more common in the | | | | lower the risk of prostate cancer. Other |
| United States). Studies have shown that | | | | examples of drugs that show promise for |
| immigrants develop the risk of their new | | | | chemoprevention include COX-2 inhibitors |
| country, suggesting a link between diet | | | | (which inhibit a cyclooxygenase enzyme |
| and cancer rather than a genetic basis. | | | | involved in the synthesis of |
| Despite frequent reports of particular | | | | proinflammatory prostaglandins). |
| substances (including foods) having a | | | | Genetic testing |
| beneficial or detrimental effect on | | | | Genetic testing for high-risk |
| cancer risk, few of these have an | | | | individuals, with enhanced surveillance, |
| established link to cancer. These | | | | chemoprevention, or risk-reducing |
| reports are often based on studies in | | | | surgery for those who test positive, is |
| cultured cell media or animals. Public | | | | already available for certain |
| health recommendations cannot be made on | | | | cancer-related genetic mutations. |
| the basis of these studies until they | | | | |