Epidemiology of cancer

The epidemiology of cancer is the investigation of the incidence and causes of the disease in people under different conditions of life. Such investigations have generally been the way in which reliable evidence about causal agents for cancer, and the magnitude of the risks from these agents, have been found. They have shown that any type of cancer that is common in one population is rare in some other, and that the differences between populations are mostly not genetic, but rather the consequences of behaviours and circumstances of life. In principle, cancers are therefore largely preventable.

The range of incidence rates between geographical and ethnic groups is more than 10-fold for each of the common cancers, and for some cancers is more than 100-fold. Large changes in rates of many tumours can occur in migrants compared with rates in their homeland, and large changes have occurred in rates within populations over time, indicating the scope for prevention.

The causes of cancer

These can be divided into nature (biological factors), nurture (environment and behaviours), and chance.

Biological factors—important biological factors are genetic susceptibility, age, and sex.

Tobacco smoking—this is the most important extrinsic factor causing cancer in developed countries, and is a major cause of cancers of the mouth, pharynx (other than nasopharynx), oesophagus, larynx, lung, pancreas, renal pelvis, and bladder (and it also causes a proportion of several other types of cancer). In 2005, smoking is estimated to have caused 28% of all fatal cancers in the United Kingdom.

Other extrinsic causes—there are many, including (1) alcohol—the cause of at least six types of cancer, including liver, various upper aerodigestive sites, and breast; (2) ionizing radiation—can cause cancer in most tissues; in the United Kingdom the main sources of exposure are natural sources including radon, and medical uses; (3) ultraviolet radiation—causes skin cancer; (4) infection, principally viral, but also bacterial and parasitic—a major cause of cancer of several sites, especially in developing countries; (5) immunosuppression—patients with persistent immunosuppression from therapeutic, infective, or genetic causes have raised risks of certain cancers, notably non-Hodgkin’s lymphoma; (6) chemotherapeutic agents—about 20 of these, used for treatment of specific diseases and including several that are used to treat cancer, have been shown themselves to cause cancer, of different anatomical sites according to the agent; (7) other drugs—hormone replacement therapy and oral contraceptives, both widely used in the general population, affect the risk of certain female reproductive-related malignancies, increasing risk for some cancers but (for combined steroid contraceptives) decreasing it for others; (8) occupation—numerous occupational groups have been found to be at raised risk of cancer, mainly of the respiratory tract, especially the lung; (9) air, water, and food pollution—these are probably responsible for a small percentage of cancers in Western countries; (10) diet—this may well have an effect on the aetiology of a substantial proportion of cancers, but there is considerable uncertainty on the figure and specific dietary associations are largely unknown.

Other factors of particular note are (1) menstrual and reproductive history, also certain hormonal drugs—these affect the risks of breast, endometrial, and ovarian cancers in women; (2) obesity—relates to increased risks of breast, endometrial, colonic, kidney, and possibly other cancers; (3) physical inactivity—relates to increased risk of breast, endometrial, colonic, and possibly other cancers.

Epidemiology and aetiology of particular cancers

The most common cancers worldwide are those of the lung, breast, and colorectum, and the most common causes of cancer death are lung, stomach, and liver cancers. Descriptive and aetiological epidemiological information is given about 33 types of cancer in this chapter.

Lung cancer—the major cause is smoking tobacco, particularly cigarettes. Lung cancer became epidemic in men in Western countries during the mid 20th century, with rates rising later in women, and in Western countries there have been considerable decreases in men in recent years. In developing countries, however, the epidemic has arrived later, with rising rates to be expected in future years as a consequence of current smoking levels. Occupational causes of lung cancer include exposures to asbestos, polycyclic hydrocarbons, and radon. Air pollution in towns may have been a factor, largely in smokers, and radon in indoor air contributes to a small percentage of cases.

Breast cancer—the most common cancer worldwide in women. It has greatest incidence in Western countries, where rates have tended to increase slowly over decades; rates have generally been much lower in Asia and Africa. Hormonal and reproductive factors are important to risk: early menopause, late menarche, nulliparity, and older age at first full-term pregnancy all increase risk, as do postmenopausal hormone replacement therapy and combined oral contraceptives, while tamoxifen treatment of unilateral breast cancer decreases risk in the unaffected breast. There is also raised risk of breast cancer in relation to a history of benign breast disease, alcohol consumption, lack of physical exercise, postmenopausal obesity, taller height, and ionizing radiation exposure at young ages, as well as genetic predisposition.