cancer explained

 
 

Cancer is the end product of a multistep process (carcinogenesis) that occurs over many years.   The term "cancer" actually refers to numerous distinct diseases characterized by abnormal cell growth and differentiation.   Cancers are categorized by the organ and/or cell of origin.   For example, squamous cell carcinoma of the lung arises from pulmonary epithelial tissue, whereas adenocarcinoma of the breast arises from mammary duct epithelium.    The natural history of a cancer is highly dependent on the organ and cell type from which it is derived.    In addition, prognosis is influenced by the stage and histologic grade of the cancer.    Staging is generally designated by the TNM (tumor, nodes, metastasis) staging system, which takes into account the size of the primary tumor(T), the extent to which local lymph nodes (glands) are involved (N), and whether or not distant metastases are present (M).    The histologic grade, determined by microscopic examination of the biopsy specimen, provides an objective assessment of the degree of cellular differentiation.
INCIDENCE, PREVALENCE, AND MORTALITY
The worldwide burden of cancer is a major health problem, with more than 8 million new cases and 5 million deaths per year.    The burden from cancer may be described in terms of incidence (number of new cases per 100,000 each year), prevalence (number of people at a given point in time with a cancer diagnosis), and mortality (number of cancer deaths).   
With few exceptions, cancer incidence, prevalence, and mortality rates are higher in industrialized countries (e.g., United States, European nations)
than in developing countries (e.g., African nations, China).    Incidence rates for specific cancers can be dramatically affected by the use of screening procedures to identify asymptomatic disease.   This is illustrated by the dramatic increase in the incidence of prostate cancer that accompanied the introduction of prostate-specific antigen (PSA) screening in the late 1980s.    Similarly, prevalence rates may be a poor index for comparing cancers, because they are dependent upon incidence, natural history, and treatment efficacy.    For example, due to the relatively short life expectancy of individuals with pulmonary neoplasms, the prevalence of lung cancer is much lower than that of prostate cancer, despite the higher mortality rates associated with lung cancer. In addition to the impact of screening and natural history, prevalence rates increase as treatment improves, because therapeutic advances enable individuals to live longer following a cancer diagnosis.
Worldwide, lung cancer is the leading cause of cancer mortality, followed by stomach cancer.    Smoking remains the leading preventable cause of cancer, and mortality and incidence rates of lung cancer rise and fall with smoking rates.    The current trend shows a leveling off of smoking-related cancers in developed countries, possibly because of health-promotion and disease-prevention efforts.    Geographical variations occur in cancer incidence and mortality, with Africa and Asia generally having lower rates than North America and Europe.  
However, it has been noted that differences in data collection and diagnostic practices make worldwide cancer comparisons somewhat difficult.   Overall, worldwide incidence rates of breast, colon and rectum, and prostate cancers are highest in developed countries, while cancers of the cervix, mouth and pharynx, esophagus, and liver are higher in developing countries.   
Migration studies generally report that migrants from developing countries to developed countries adopt cancer incidence rates equivalent to those of their new country.   For example, studies of Japanese and Chinese immigrants living in the United States show that their risks for prostate and breast cancers increase dramatically the longer they reside in the United States.
Similar trends for increased risk are seen among African immigrants in European countries. Geographical variations in cancer incidence and mortality also exist in the United States.
Each cancer site shows some variation, such as higher prostate cancer mortality rates in the South Central and southern Atlantic states, and higher breast cancer mortality rates in the northeastern states.   The Atlas of United States Mortality, published by the Centers for Disease Control and Prevention, provides detailed geographic information on cancer mortality rates in the United States, and is available online at http://www.cdc.gov/nchs/data/atlasmet.pdf.
Lung cancer is the leading cause of death from cancer for men and women, accounting for almost one-third of cancer deaths.
 
Exposure to carcinogens in tobacco smoke accounts for almost one-third of cancer cases, especially cancers of the lung, respiratory tract, esophagus, bladder, pancreas, and, most likely, cancers of the stomach, liver, and kidneys.   
Carcinogens found in the environment and the workplace (e.g., asbestos, benzene, vinyl chloride compounds, dyes, arsenic, petroleum products) and cancers associated with exposure to these chemicals (e.g., lung and bladder) are higher in urban areas than in rural areas.    Have you been poisoned or is there poison in anything around you?    
Diet also influences the risk of cancer, although researchers are unsure of the mechanisms involved. In general, evidence supports an increased risk of various cancers (e.g., colon, rectum) with a high intake of red meats, and a decreased risk of various cancers (e.g., lung, colon, stomach) with a high intake of vegetables and fruits.    Other food constituents, such as vitamins and minerals, are also being investigated for their ability to prevent cancer.
 
Other possible causes of cancer include pathogens, such as hepatitis B and C viruses in liver cancer, and the Helicobacter pylori bacterium in stomach cancer. Hormonal factors contributing to cancer have focused on estrogen, progesterone, and testosterone, and their role in reproductive organ cancers.