Definitions of what risk factors are modifiable vary, but consensus estimates in the 1980s, 1990s, and the 2000s agree that the majority of cancers are theoretically preventable by various combinations of risk factor reduction, and immunization against cancer-causing infectious agents.
Primary prevention refers to the strategy of reducing risk factor prevalence, and thus reducing the incidence of cancer. Reducing the prevalence of smoking and the use of other tobacco products is the single factor with the greatest potential for reducing cancer risk. Unfortunately, while per capita consumption of cigarettes has approximately halved in several high-income countries since the 1970s, global consumption is still increasing, fuelled by younger age population structures in less developed countries, as well as the tobacco industry’s marketing in countries where cigarette consumption was previously low.
Next, to smoking, there is a consensus that a diet that minimizes weight gain through the course of life is the next major potentially preventive factor. Although the precise content of the diet that is preventive remains unclear, consensus panels stress diets high in fruits and vegetables and low in meat and processed meat products. Minimal to moderate alcohol exposure reduces cancer risk.
Avoidance of sunburns and long periods of ultraviolet exposure will reduce the risk of skin cancers. Hepatitis B and HPV vaccination are underused strategies globally; clinical interventions against H. pylori would probably reduce the risk of stomach cancer, but this is unproven. Occupational health regulations and enforcement are needed to reduce the risk of exposure to ionizing radiation and workplace carcinogens. Reduction in use of postmenopausal hormones has already been shown to reduce rates of oestrogen-positive breast cancer. Reproductive factors such as age at menarche, parity, and age at first birth, although theoretically modifiable, are difficult to change (e.g., extreme physical activity delays menarche) or are socially determined in a way that tends to supersede concerns about future cancer risk.
Secondary prevention aims to prevent cancer death in those diagnosed with cancer or a premalignant lesion, usually by treating at an early stage. The major success in cancer screening has been organized cervical cancer screening, which is responsible for a dramatic reduction in cervical cancer deaths in countries such as the United States, where most cases occur in women with a history of suboptimal screening. A decline in U.S. colorectal cancer incidence is at least partly due to screening for early cancers and premalignant colorectal adenomas, with faecal occult blood testing, sigmoidoscopy, and colonoscopy having different trade-offs in terms of patient acceptance and test performance. The use of PSA screening for prostate cancer and mammography for breast cancer remains controversial. In both cases, there is plausibly a reduction in the death rate from each cancer in men or women screened, respectively. However, the better appreciation of the harms of screening and “overdiagnosis” (i.e., detection and treatment of lesions that might never have come to clinical attention) has meant that recommendations have been changed in recent years. In the case of breast cancer, better treatment of the disease and better awareness by women of the need to seek early assessment of any lumps or changes in the breast may mean that the effectiveness of mammography in reducing death rates has changed over time. A report for the U.S. Preventive Services Taskforce describes the evidence as “strong” that low-dose computed tomography screening reduces the risk of lung cancer and lung cancer death, although the evidence is based on a single, large, good-quality study.
Although many cancers can be cured if detected early enough, the sheer volume of cancers in less developed countries with limited infrastructure, plus the cost of frequent screening, means that organized screening activities will take time to develop. The exception may be cervical screening, for which HPV testing combined with gynaecologic follow-up may provide a means of screening large numbers of women relatively rapidly, although this is by no means a trivial endeavour. Greater awareness of cancer symptoms and signs, combined with appropriate and prompt follow-up, will remain a mainstay of attempts to downstage cancers in less developed regions, and developing adequate diagnostic and treatment services may be required before instituting screening services.