Epidemiology of cancer
The epidemiology of cancer is the study of the factors affecting cancer, as a way to infer possible trends and causes. The study of cancer epidemiology uses epidemiological methods to find the etiology (cause) of cancer and to identify and develop improved treatments.
This area of study must contend with problems of lead time bias and length time bias. Lead time bias is the concept that early diagnosis may artificially inflate the survival statistics of a cancer, without really improving the natural history of the disease. Length bias is the concept that slower growing, more indolent tumors are more likely to be diagnosed by screening tests, but improvements in diagnosing more cases of indolent cancer may not translate into better patient outcomes after the implementation of screening programs. A similar epidemiological concern is overdiagnosis, the tendency of screening tests to diagnose diseases that may not actually impact the patient's longevity. This problem especially applies to prostate cancer and PSA screening.[1]
Some cancer researchers have argued that negative cancer clinical trials lack sufficient statistical power to discover a benefit to treatment. This may be due to fewer patients enrolled in the study than originally planned.[2]
Risk factors
Over a third of cancer deaths worldwide are due to potentially modifiable risk factors. The leading modifiable risk factors worldwide are tobacco smoking, alcohol use, and diet low in fruit and vegetables; in developed countries overweight and obesity is also a leading cause of cancer, and in low-and-middle-income countries sexual transmission of human papillomavirus is a leading risk factor for cervical cancer. Men with cancer are twice as likely as women to have a modifiable risk factor for their disease.[8]
The vast majority of cancer risk factors are environmental or lifestyle-related in nature, leading to the claim that cancer is a largely preventable disease.[8] Examples of modifiable cancer risk factors include alcohol consumption (associated with increased risk of oral, esophageal, breast, and other cancers), smoking (although 20% of women with lung cancer have never smoked, versus 10% of men[9]), physical inactivity (associated with increased risk of colon, breast, and possibly other cancers), and being overweight (associated with colon, breast, endometrial, and possibly other cancers). Based on epidemiologic evidence, it is now thought that avoiding excessive alcohol consumption may contribute to reductions in risk of certain cancers; however, compared with tobacco exposure, the magnitude of effect is modest or small and the strength of evidence is often weaker. Other lifestyle and environmental factors known to affect cancer risk (either beneficially or detrimentally) include certain sexually transmitted diseases (such as those conveyed by the human papillomavirus), the use of exogenous hormones, exposure to ionizing radiation and ultraviolet radiation, and certain occupational and chemical exposures.
Every year, at least 200,000 people die worldwide from cancer related to their workplace.[10] Millions of workers run the risk of developing cancers such as lung cancer and mesothelioma from inhaling asbestos fibers and tobacco smoke, or leukemia from exposure to benzene at their workplaces.[10] Currently, most cancer deaths caused by occupational risk factors occur in the developed world.[10] It is estimated that approximately 20,000 cancer deaths and 40,000 new cases of cancer each year in the U.S. are attributable to occupation.[11]
Incidence and mortality
In the U.S. cancer is second only to cardiovascular disease as the leading cause of death;[12] in the UK it is the leading cause of death.[13] In many Third World countries cancer incidence (insofar as this can be measured) appears much lower, most likely because of the higher death rates due to infectious disease or injury. With the increased control over malaria and tuberculosis in some Third World countries, incidence of cancer is expected to rise; this is termed the epidemiologic transition in epidemiological terminology.
Cancer epidemiology closely mirrors risk factor spread in various countries. Hepatocellular carcinoma (liver cancer) is rare in the West but is the main cancer in China and neighbouring countries, most likely due to the endemic presence of hepatitis B and aflatoxin in that population. Similarly, with tobacco smoking becoming more common in various Third World countries, lung cancer incidence has increased in a parallel fashion.
Cancer is responsible for about 25% of all deaths in the U.S., and is a major public health problem in many parts of the world. The statistics below are estimates for the U.S. in 2008, and may vary substantially in other countries. They exclude basal and squamous cell skin cancers, and carcinoma in situ in locations other than the urinary bladder.[12]
Male
most common (by occurrence) most common (by mortality)[12]
- prostate cancer (25%) lung cancer (31%)
- lung cancer (15%) prostate cancer (10%)
- colorectal cancer (10%) colorectal cancer (8%)
- bladder cancer (7%) pancreatic cancer (6%)
- non-Hodgkin lymphoma (5%) liver & intrahepatic bile duct (4%)
Female
most common (by occurrence) most common (by mortality)[12]
- breast cancer (26%) lung cancer (26%)
- lung cancer (14%) breast cancer (15%)
- colorectal cancer (10%) colorectal cancer (9%)
- endometrial cancer (7%) pancreatic cancer (6%)
- non-Hodgkin lymphoma (4%) ovarian cancer (6%)
Child cancers
Cancer can also occur in young children and adolescents, but it is rare (about 150 cases per million yearly in the US). Leukemia (usually acute lymphoblastic leukemia) is the most common cancer in children aged 1–14 in the U.S., followed by the central nervous system cancers, neuroblastoma, Wilms' tumor, and non-Hodgkin's lymphoma.[12] Statistics from the SEER program of the US NCI demonstrate that childhood cancers increased 19% between 1975 and 1990, mainly due to an increased incidence in acute leukemia. Since 1990, incidence rates have decreased.[14]
Children living near nuclear facilities face an increased risk of cancer.[15]
Infant cancers
The age of peak incidence of cancer in children occurs during the first year of life, in infants. The average annual incidence in the United States, 1975-1995, was 233 per million infants.[14] Several estimates of incidence exist. According to SEER,[14] in the United States:
- Neuroblastoma comprised 28% of infant cancer cases and was the most common malignancy among these young children (65 per million infants).
- The leukemias as a group (41 per million infants) represented the next most common type of cancer, comprising 17% of all cases.
- Central nervous system malignancies comprised 13% of infant cancer, with an average annual incidence rate of nearly 30 per million infants.
- The average annual incidence rates for malignant germ cell and malignant soft tissue tumors were essentially the same at 15 per million infants. Each comprised about 6% of infant cancer.
Teratoma (a germ cell tumor) often is cited as the most common tumor in this age group, but most teratomas are surgically removed while still benign, hence not necessarily cancer. Prior to the widespread routine use of prenatal ultrasound examinations, the incidence of sacrococcygeal teratomas diagnosed at birth was 25 to 29 per million births.
Female and male infants have essentially the same overall cancer incidence rates, a notable difference compared to older children.
White infants have higher cancer rates than black infants. Leukemias accounted for a substantial proportion of this difference: the average annual rate for white infants (48.7 per million) was 66% higher than for black infants (29.4 per million).[14]
Relative survival for infants is very good for neuroblastoma, Wilms' tumor and retinoblastoma, and fairly good (80%) for leukemia, but not for most other types of cancer.