Cancer

A coronal CT scan showing a cancer of right pleural membranes, the outer surface of the lung and inner surface of the chest wall, malignant mesothelioma.
Legend: → tumor ←, ★ central pleural effusion, 1 & 3 lungs, 2 spine, 4 ribs, 5 aorta, 6 spleen, 7 & 8 kidneys, 9 liver

Cancer (medical term: malignant neoplasm) is a class of diseases in which a group of cells display uncontrolled growth, invasion that intrudes upon and destroys adjacent tissues, and sometimes metastasis, or spreading to other locations in the body via lymph or blood. These three malignant properties of cancers differentiate them from benign tumors, which do not invade or metastasize.

Researchers divide the causes of cancer into two groups: those with an environmental cause and those with a hereditary genetic cause. Cancer is primarily an environmental disease, though genetics influence the risk of some cancers. Common environmental factors leading to cancer include: tobacco, diet and obesity, infections, radiation, lack of physical activity, and environmental pollutants. These environmental factors cause or enhance abnormalities in the genetic material of cells. Cell reproduction is an extremely complex process that is normally tightly regulated by several classes of genes, including oncogenes and tumor suppressor genes. Hereditary or acquired abnormalities in these regulatory genes can lead to the development of cancer. A small percentage of cancers, approximately five to ten percent, are entirely hereditary.

The presence of cancer can be suspected on the basis of symptoms, or findings on medical imaging. Definitive diagnosis of cancer, however, requires the microscopic examination of a biopsy specimen. Most cancers can be treated. Possible treatments include chemotherapy, radiotherapy and surgery. The prognosis is influenced by the type of cancer and the extent of disease. While cancer can affect people of all ages, and a few types of cancer are more common in children, the overall risk of developing cancer increases with age. In 2007 cancer caused about 13% of all human deaths worldwide (7.9 million). Rates are rising as more people live to an old age and lifestyles change in the developing world.

Classification

Cancers are classified by the type of cell that the tumor resembles and is therefore presumed to be the origin of the tumor. These types include:

• Carcinoma: Cancer derived from epithelial cells. This group includes many of the most common cancers, including those of the breast, prostate, lung and colon.
• Sarcoma: Cancer derived from connective tissue, or mesenchymal cells.
• Lymphoma and leukemia: Cancer derived from hematopoietic (blood-forming) cells
• Germ cell tumor: Cancer derived from pluripotent cells. In adults these are most often found in the testicle and ovary, but are more common in babies and young children.
• Blastoma: Cancer derived from immature "precursor" or embryonic tissue. These are also commonest in children.

Cancers are usually named using -carcinoma, -sarcoma or -blastoma as a suffix, with the Latin or Greek word for the organ or tissue of origin as the root. For example, a cancer of the liver is called hepatocarcinoma; a cancer of fat cells is called a liposarcoma. For some common cancers, the English organ name is used. For example, the most common type of breast cancer is called ductal carcinoma of the breast. Here, the adjective ductal refers to the appearance of the cancer under the microscope, which suggests that it has originated in the milk ducts.

Benign tumors (which are not cancers) are named using -oma as a suffix with the organ name as the root. For example, a benign tumor of smooth muscle cells is called a leiomyoma (the common name of this frequently occurring benign tumor in the uterus is fibroid). Confusingly, some types of cancer also use the -oma suffix, examples including melanoma and seminoma.

Signs and symptoms

Symptoms of cancer metastasis depend on the location of the tumor.

Cancer symptoms can be divided into three groups:

• Local symptoms: are restricted to the site of the primary cancer. They can include lumps or swelling (tumor), hemorrhage (bleeding from the skin, mouth or anus), ulceration and pain. Although local pain commonly occurs in advanced cancer, the initial swelling is often painless.
• Metastatic symptoms: are due to the spread of cancer to other locations in the body. They can include enlarged lymph nodes (which can be felt or sometimes seen under the skin), hepatomegaly (enlarged liver) or splenomegaly (enlarged spleen) which can be felt in the abdomen, pain or fracture of affected bones, and neurological symptoms.
• Systemic symptoms: occur due to distant effects of the cancer that are not related to direct or metastatic spread. Some of these effects can include weight loss (poor appetite and cachexia), fatigue, excessive sweating (especially night sweats), anemia (low red blood cell count) and other specific conditions termed paraneoplastic phenomena. These may be mediated by immunological or hormonal signals from the cancer cells.

Causes

Cancers are primarily an environmental disease with 90-95% of cases attributed to environmental factors and 5-10% due to genetics. Environmental, as used by cancer researchers, means any cause that is not genetic. Common environmental factors that contribute to cancer death include: tobacco (25-30%), diet and obesity (30-35%), infections (15-20%), radiation (both ionizing and non ionizing, up to 10%), stress, lack of physical activity, and environmental pollutants.

The incidence of lung cancer is highly correlated with smoking.

Cancer pathogenesis is traceable back to DNA mutations that impact cell growth and metastasis. Substances that cause DNA mutations are known as mutagens, and mutagens that cause cancers are known as carcinogens. Particular substances have been linked to specific types of cancer. Tobacco smoking is associated with many forms of cancer, and causes 90% of lung cancer.

Many mutagens are also carcinogens, but some carcinogens are not mutagens. Alcohol is an example of a chemical carcinogen that is not a mutagen. In Western Europe 10% of cancers in males and 3% of cancers in females are attributed to alcohol.

Decades of research has demonstrated the link between tobacco use and cancer in the lung, larynx, head, neck, stomach, bladder, kidney, esophagus and pancreas. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons. Tobacco is responsible for about one in three of all cancer deaths in the developed world, and about one in five worldwide. Lung cancer death rates in the United States have mirrored smoking patterns, with increases in smoking followed by dramatic increases in lung cancer death rates and, more recently, decreases in smoking followed by decreases in lung cancer death rates in men. However, the numbers of smokers worldwide is still rising, leading to what some organizations have described as the tobacco epidemic.

Cancer related to one's occupation is believed to represent between 2–20% of all cases. Every year, at least 200,000 people die worldwide from cancer related to their workplace. Currently, most cancer deaths caused by occupational risk factors occur in the developed world. 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. 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. 

Prevention

Cancer prevention is defined as active measures to decrease the incidence of cancer. The vast majority of cancer risk factors are environmental or lifestyle-related, thus cancer is largely a preventable disease. Greater than 30% of cancer is preventable via avoiding risk factors including: tobacco, overweight or obesity, low fruit and vegetable intake, physical inactivity, alcohol, sexually transmitted infections, and air pollution.

Dietary

Dietary recommendations to reduce the risk of developing cancer, including: (1) reducing intake of foods and drinks that promote weight gain, namely energy-dense foods and sugary drinks, (2) eating mostly foods of plant origin, (3) limiting intake of red meat and avoiding processed meat, (4) limiting consumption of alcoholic beverages, and (5) reducing intake of salt and avoiding mouldy cereals (grains) or pulses (legumes).

Proposed dietary interventions for cancer risk reduction generally gain support from epidemiological association studies. Examples of such studies include reports that reduced meat consumption is associated with decreased risk of colon cancer, and reports that consumption of coffee is associated with a reduced risk of liver cancer. Studies have linked consumption of grilled meat to an increased risk of stomach cancer, colon cancer, breast cancer, and pancreatic cancer, a phenomenon which could be due to the presence of carcinogens in foods cooked at high temperatures. Whether reducing obesity in a population also reduces cancer incidence is unknown. Some studies have found that consuming lots of fruits and vegetables has little if any effect on preventing cancer. A 2005 secondary prevention study showed that consumption of a plant-based diet and lifestyle changes resulted in a reduction in cancer markers in a group of men with prostate cancer who were using no conventional treatments at the time. These results were amplified by a 2006 study. Over 2,400 women were studied, half randomly assigned to a normal diet, the other half assigned to a diet containing less than 20% calories from fat. The women on the low fat diet were found to have a markedly lower risk of breast cancer recurrence, in the interim report of December, 2006.

Medication

The concept that medications could be used to prevent cancer is an attractive one, and many high-quality clinical trials support the use of such chemoprevention in defined circumstances. Aspirin has been found to reduce the risk of death from cancer. Daily use of tamoxifen or raloxifene has been demonstrated to reduce the risk of developing breast cancer in high-risk women by about 50%.Finasteride has been shown to lower the risk of prostate cancer, though it seems to mostly prevent low-grade tumors. The effect of COX-2 inhibitors such as rofecoxib and celecoxib upon the risk of colon polyps have been studied in familial adenomatous polyposis patients and in the general population. In both groups, there were significant reductions in colon polyp incidence, but this came at the price of increased cardiovascular toxicity.

Vitamins have not been found to be effective at preventing cancer, although low levels of vitamin D are correlated with increased cancer risk. Whether this relationship is causal and vitamin D supplementation is protective is yet to be determined. Beta-carotene supplementation has been found to increase slightly, but not significantly, risks of lung cancer. Folic acid supplementation has not been found effective in preventing colon cancer and may increase colon polyps.

Vaccination

Vaccines have been developed that prevent some infection by some viruses that are associated with cancer, and therapeutic vaccines are in developmentto stimulate an immune response against cancer-specific epitopes. Human papillomavirus vaccine (Gardasil and Cervarix) decreases the risk of developing cervical cancer. The hepatitis B vaccine prevents infection with hepatitis B virus and thus decreases the risk of liver cancer.

Advances in cancer research have made a vaccine designed to prevent cancers available. In 2006, the U.S. Food and Drug Administration (FDA) approved a human papilloma virus vaccine, called Gardasil. The vaccine protects against 6,11,16,18 strains of HPV, which together cause 70% of cervical cancers and 90% of genital warts. It also lists vaginal and vulvar cancers as being protected. In March 2007, the US Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) officially recommended that females aged 11–12 receive the vaccine, and indicated that females as young as age 9 and as old as age 26 are also candidates for immunization. There is a second vaccine from Cervarix which protects against the more dangerous HPV 16,18 strains only. In 2009, Gardasil was approved for protection against genital warts. In 2010, the Gardasil vaccine was approved for protection against anal cancer for males and reviewers stated there was no anatomical, histological or physiological anal differences between the genders so females would also be protected.