Incidence and Distribution of Cancers

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Cancer epidemiology is the study of the incidence (number of newly diagnosed cases), prevalence (number of existing cases), and mortality (number of deaths) of specific diseases in defined populations during specific periods of time. The ultimate aim is the determination of risk factors for individual cancers with respect to geographic, demographic, socioeconomic, behavioral, and genetic patterns. Descriptive epidemiology studies are used to determine a possible cause for a particular cancer and may be refined by inclusion of the time factor, such as cross-sectional studies (present time), case-control studies of past exposure (retrospective studies), and attempts to establish if selected exposed populations develop a disease in the future (prospective studies). The discipline of molecular epidemiology incorporates molecular, cellular, and other biological measurements into epidemiological research.

According to the World Health Organization, in 1997, ~10 million people were diagnosed with cancer and ~6 million died from the disease worldwide. In the U.S., cancer is the second most common cause of death among adults (~24% of total deaths) following heart disease (~33%). About 1500 people die of cancer every day. In both females and males, more than one-half of cancer deaths are attributable to four malignancies. For females they are lung (25%), breast (17%), and colon (10%), while for males the distribution is lung (32%), prostate (14%), and colon (9%).

The incidence of specific cancers varies significantly across the world, possibly due to different environmental factors. For example, the incidence of melanoma is ~150-fold greater in Australia than in Japan, prostate cancer occurs ~70 times more frequently in the U.S. than in China, and liver cancer is ~50 times more prevalent in China than in Canada. At the same time, the incidences of certain malignancies such as leukemia, ovarian cancer, and breast cancer are similar in most countries. Even within these general differences, the use of cancer maps has, in certain instances, disclosed some dramatic variations that are likely to be related to local socioeconomic or environmental factors.

The lifetime probability of developing a malignancy is ~1 in 2 for males and ~1 in 3 for females. These numbers increase steadily with age. For example, for males in the 40 to 59-year age bracket there is a 1 in 78 probability for occurrence of prostate cancer which increases to 1 in 6 in the 60 to 79-year-old group. Similarly, for females, the probability for breast cancer in the same age brackets increases from 1 in 26 to 1 in 14.

Survival analysis provides the link between incidence and mortality. Although it has no particular biological significance, the 5-year survival rate has become the standard outcome measure in the clinical literature. A hundred years ago, cancer was invariably fatal. Today, in the U.S. the overall 5-year survival rate for all cancers is ~55% (by race 58% white, 42% black). There have been spectacular advances in the early detection and treatment of some uncommon cancers, such as childhood leukemia and testicular cancer. The 5-year survival for advanced stages of common malignancies has also improved by 2 to 10% during the past decade.

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