Cancer

Cancer, group of diseases in which some body cells change their nature, start to divide uncontrollably, and may revert to an undifferentiated type. They form a malignant tumor that enlarges and may spread to adjacent tissues or through blood and lymph systems to other parts of the body. The American Cancer Society has listed 7 warning signals: (1) change in bowel or bladder habits, (2) a sore that does not heal, (3) unusual bleeding or discharge, (4) thickening or lump in a breast or elsewhere, (5) indigestion or difficulty in swallowing, (6) obvious change in a wart or mole, and (7) nagging cough or hoarseness. Every diagnosis of cancer made by a doctor is subject to confirmation by a biopsy, in which a small piece of tissue is cut into very thin slices, stained with special dyes, and examined under a microscope. Routine periodic physical examinations are important, since the most successfully treated cancers are those that have not progressed to the stage of producing symptoms. The doctor feels, or palpates, parts of the body for unusual lumps or thickenings. A cervical smear is taken from adult women. The rectum and large intestine can be examined by means of a lighted tube, called a proctosigmoidoscope. Periodic self-examination of the skin, mouth, and genital organs is also important. Women should examine their breasts once a month after their menstrual cycles, and men should examine their testicles.

Successful treatment of cancer requires complete removal or destruction of the tumor by surgery, radiation, or drugs (chemotherapy). Some tumors cannot be destroyed by safe amounts of radiation, and some cannot be entirely removed surgically without destroying a vital organ, so surgery, radiation, and drugs may be combined.

Cancer is tenacious, sometimes subsiding but reappearing years after the patient has presumably been cured. For some cancers, even 5 years is not a long enough period; an indefinite length of time with frequent examinations is recommended. Physicians are increasingly reluctant to speak of “5-year cures,” preferring more the realistic expression of “5-year survival without clinical evidence of disease.”

Causes of cancer

Chemical compounds

Chemists have manufactured several hundred pure chemicals that produce cancer in animals. In humans, however, research has not found a relationship between chemical structure and cancer-producing activity. Some chemical carcinogens produce cancers at the site of contact, others at distant sites. Some are incomplete or weak in their action and need the action of other chemicals in order to induce tumors. The simultaneous injection of a chemical carcinogen and a closely related inactive compound (anticarcinogen) can prevent or retard the action of the carcinogen. This line of investigation may eventually uncover chemicals to prevent the development of cancer in individuals who are unavoidably exposed to certain cancer-producing hazards.

Environmental hazards

Molds and bacteria growing naturally on food products may represent a source of environmental carcinogens. The human cancers due to industrial exposure include the following: (1) bladder cancer in aniline dye workers who handle betanaphtylamine, (2) bone cancer due to swallowing radium, (3) lung cancer caused by inhalation of chromium compounds, radioactive ores, asbestos, arsenic, and iron, (4) cancer of nasal sinuses and the lung in nickel mine workers, and (5) skin cancer from the handling of some products of coal, oil shale, lignite, and petroleum. Excessive or continuous exposure to inhaled impurities brings about changes in the bronchial linings and the lungs that may eventually result in disability and illness. If the impurities contain cancer-producing substances, prolonged exposure can lead to cancer. During the past 40 years, cancer of the lung in the United States has shown the greatest increase of any cancer type. At least 80% of the total increase can be attributed to cigarette smoking. An additional important factor is air pollution caused by industrial wastes, car exhausts, and household sources.

Radiation

The cancer-producing effects of the ultraviolet rays of sunlight appear to be limited to the skin. Related to the cancer-causing effects of sunlight are those of ionizing radiation from radium and X-rays. Ionizing radiation can cause several forms of cancer in humans and animals. Radiologists and others exposed to increased doses of radiation are more likely to develop leukemia than are people who are not exposed. Radium salts, which are deposited in bone, give rise to cancers of the bone.

Cancer-causing viruses.

Cancer-causing viruses penetrate the cells of victims and become part of the structure of the chromosomes. The presence of the virus in the genetic mechanism of the cell may make the cell behave immediately as a cancer, or the virus can remain dormant until activated by another stimulus. Scientists have identified several dozen types of cancer that are caused by viruses, in many species of animals, including cats, monkeys, chickens, and rodents. It is inevitable that some human cancers will be found to be caused by viruses and that such discoveries may lead to the development of protective vaccines.

Hormones

Large doses of the female hormone estrogen given over long periods will lead to the development of leukemia and tumors of the testes, uterus, and pituitary in some strains of mice. The increasing use of hormones as treatment for various human conditions does not seem to have led to an increase in the incidence of any specific type of cancer in women or men. However, some girls born of mothers who took large doses of a synthetic estrogen, diethylstilbestrol (DES), during their pregnancy have developed cancer of the vagina. Widespread use of “the pill” (the collective term for a variety of oral contraceptives containing progestogen, often estrogen, and sometimes other hormones) has increased the number of younger women receiving additional hormones. Carefully controlled, long-term observations of “pill” users are necessary to assess increased risks for cancer of the breast, uterus, or ovary. Established tumors in both humans and laboratory animals have shown varying degrees of hormone dependency. For example, removal of the ovaries in some women with advanced breast cancer or removal of the testes in men with prostate cancer often leads to temporary regression of the tumors.

Nutrition

There is no diet known to prevent cancer in humans. There is some evidence, that vitamin deficiency in humans plays a role in the occurrence of cancers of the mouth and the esophagus. However, such a deficiency is probably only 1 of a number of factors. The consumption of dietary fiber has been associated with reduced risk of colon cancer.

Heredity.

The limited data available suggest increased familial risk of developing cancer of the same site for cancers of the breast (female), stomach, large intestine, endometrium (lining of the uterus), prostate, lung, and possibly ovary. However, it is not known to what extent the tumors that have been observed to run in some families are due to genetic characteristics or to environmental factors such as diet or occupation that may remain the same from one generation to the next. Brain tumors and sarcomas seem to occur more frequently than expected in brothers and sisters of children with these tumors. When an identical twin has childhood leukemia, the probability that the other twin will develop the disease within 1 or 2 years of the date of diagnosis of the first twin is about 1 in 5. In addition, retinoblastoma, a rare form of cancer of the eye, is known to be due to inherited mutation.