The diseases grouped under cancer are second only to another group of diseases, of the heart and blood vessels, as killers of the people of the United States. This is also true of European and other countries that have kept pace with modern public health technology, and have reduced tuberculosis, other infectious diseases and malnutrition to relatively unimportant roles.
In numbers, among the 180 million people in the United States of 1960, approximately a half million develop cancer during one year, and approximately 280,000 die of cancer during one year. It is more convenient to express these facts in ratios, using 100, 000 people as a standard number and a year as a standard time period. We would then say that the annual crude incidence and mortality rates of cancer are 275 and 155 per 100,000, respectively.
We should note that these are “crude” rates. By this is meant that they have not been adjusted for the age distribution of the population, which is of great importance in considering diseases that occur in higher frequency among older members of the population. There has been a steady, striking rise in the number of cancer deaths in the United States during the past 50 years. Some of this rise is undoubtedly due to better diagnosis of cancer, and better reporting of cancer cases. But a much more important factor has been the increasingly older population, saved from earlier death from other causes. If this aging is taken into consideration, and statistics from different years adjusted to a similar age distribution, relatively little change in the incidence and mortality from cancer remains.
Immediately we must specify that this is by no means true of all cancer entities. Some remarkable changes have occurred in certain types of cancer, and these changes provide us with clues regarding causation that may lead us to eventual control over these diseases. The most striking change has been observed in the increase of lung cancer among men. This cancer, relatively rare only 30 years ago, is now among the top three cancer killers of men. Extensive studies relate this tragic rise to environmental inhalation of cancer-info-producing chemicals, derived from tobacco products and from our increasingly contaminated atmosphere.
As interesting and as important as the rise in lung cancer has been the impressive drop in the death rate from cancer of the stomach. During the past 30 years, in women as well as in men, this cancer entity has dropped in occurrence by about 50 percent. We do not understand the reasons for this happy situation in the United States. Cancer of the stomach does seem to be a disease of underprivilege and cold climate, occurring more frequently among the lower socioeconomic classes, and among the people of Iceland, Scandinavia, Russia and Poland, and Japan. Epidemiologic and laboratory studies focused toward a more specific definition of the factors that are involved may provide us with leads for the further reduction of this important form of cancer.
In the United States, the last 30 years also have shown a steady increase in the reported deaths from leukemia and related cancers of the blood and lymphatic systems, and a decrease in mortality from cancer of the uterine cervix in women. Part of the latter effect is due to better treatment., The salvage rate of women with cancer of the uterus and cervix is improving continually, particularly as a greater proportion of the cases is detected at earlier stages by the use of the vaginal smear technic introduced by the late George N. Papanicolaou of Cornell University Medical College.
In the United States and many European countries, the most common forms of cancer among women are cancers of the breast, the uterus, and the large intestine. Among men, the lung, prostate and large intestine are among the most frequent sites in the death statistics. In terms of incidence, skin cancer ranks high among fair-skinned males, but this is not apparent in the death registries because most patients with skin cancer are successfully treated.
So far we have discussed primarily the population of the United States. What about the rest of the world population, which is rapidly approaching 4 billion people?
Careful studies going back to the last century show that human beings of all races, colors, and habitations develop cancer of one type or another. It is true that the relative frequency of different types of cancer do differ very significantly in diferent locations of the world. However, reports of primitive peoples that are “immune” to cancer uniformly have proved to be wrong. Primitive groups, whether they be Eskimos or South American tribes, usually consist of small numbers of people, who die at young ages and who lack medical care. Mice captured in the home or in the fields also very rarely show cancers. Yet the same animals, allowed to live their full life spans in the laboratory and carefully studied, develop as many and as varied a spectrum of cancers as human beings. An analogous human situation may be seen if observations are made on a group of workers engaged in an occupation requiring health and physical strength. These would be found to be young and having less disease than the general population, yet at later ages they may well develop many more disabilities due to their occupational hazards.
There are some interesting differences in occurrence of different cancers among human populations that inhabit the earth. We have mentioned the high frequency of stomach cancer along a great northern belt extending from Iceland to Japan. Cancer of the uterine cervix is preeminent among females of India and China, and in the latter country cancer of the penis is a common disease. Cancer of the cervix and of the penis are also frequent in many South American countries, and in other countries with relatively poor standards of personal hygiene. Cancer of the liver is several times more frequent among peoples of the sub-Sahara belt of Africa and in Indonesia than it is in Europe and in the Americas. Cancer of the skin is less than one-fourth as frequent among Negroes as among the so called white races. In Japan, cancer of the breast in women, and cancer of the prostate in men are one-fourth as frequent as in the United States.
Cancer has been observed in all species of vertebrate animals that have been studied adequately, meaning that careful autopsy examinations were carried out on, say, several hundred of such animals reaching old age for their species. In fact, cellular abnormalities that resembled cancer are observed in plants, such as the crown-gall of tomatoes. Cell masses that resemble cancers of higher animals have been studied and produced also in insects.
Fish develop cancers identical in appearance and in behavior to those seen in man. During the past several years, it has been noted that rainbow trout in the United States developed a high frequency of liver cancers (hepatomas) .
Among the amphibians, the common frog, Rana pipiens, develops kidney cancers, which may be due to a virus-like agent. Birds have a wide variety of cancers, and one of the most important is the leukemia complex of chickens. In the United States, practically all flocks are infected with the virus entities causing not only leukemia of several types, but also cancers of the connective tissue, called sarcomas.
A variant of leukemia, usually localized to lymph glands, is also an important disease among cattle. The white-faced Here-ford cattle develop cancer of the skin around the eye following exposure to the intense ultraviolet radiation of our deserts and plains. It is interesting that cows practically never have cancer of the breast or the udder. Among horses, cancer of the skin and of the penis are among the common forms.
The favorite pet animal of the American people is the dog, and there are extensive observations on very old dogs. They show that dogs are prone to develop cancer of the breast, testis, adrenal gland, and leukemia. In some localities of the southeastern United States, dogs have cancer of the esophagus following infestation by a small worm that lodges in the surrounding tissue. There is a venereal sarcoma in dogs that is transmitted by sexual or oral contact, and that has an interesting characteristic of regressing and healing in a significant proportion of the afflicted animals.
Cats, too, have cancers, although not in as high a frequency as dogs. Among the common feline tumors are leukemias and breast cancers.
Only a few species of monkeys and anthropoid apes have been studied adequately, but it does seem that the common rhesus monkey develops relatively few cancers. Nevertheless, when these primates are allowed to live out their life span, some cancers have been found, and bone cancers have been produced by implanting radium into bone.
It appears to be a sound generalization to conclude that all multicellular animals, and certainly all vertebrates, can develop cancer. Thus, the ability to develop into cancer is a characteristic of all cells that are capable of reproduction and growth.
Until the recent resurgence of interest in the virus aspects of cancer, the description of cancers in animals was relegated to comparative, anecdotal pathology. It is now obvious that there must be some important albeit subtle interrelationships between the cancers of man and the cancers in animals that inhabit the same world. The nature of this interrelationship remains to be clarified, and it is probably multiple and complex. For example, cancer of the mouth occurs in fish that live in harbor waters contaminated by tars and oils. These fish are therefore exposed to environmental cancer-info-producing hazards that may also involve man. In other situations, such as the leukemias of chickens, cows, mice, dogs, and man, a related group of viral entities with undetermined vectors, carriers and transmitters between several species must be considered, and is no longer too farfetched a possibility for research exploration.
Cancer of the breast and of other sites has been described since the dawn of medicine. A hemangioma, a tumor arising from blood vessels, was found in the tail bones of a dinosaur. Several Egyptian mummies are known with bone changes indicating the presence of cancer. Thus, although we have many established examples of cancers due to occupational and other environmental exposures, cancer as a group cannot be attributed to the products of human creation called civilization.