Ultraviolet radiation and skin cancer in man

PREVENT,VE

MEDICINE

9, 227-230

Ultraviolet

(1980)

Radiation

and Skin

COMMITTEE JOHN C. BAILAR,

111, M.D.,

Lkpartt?lrtlt

DEMOPOULOS,

in Man’

MEMBERS

PH.D.

GIO B. GORI,

Jor~ruul oft/w NC-I, Nuriond Cancer Imtitute HARRY

Cancer

P1i.D.

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M.D.

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Skin cancers are the most common malignant tumors of white-skinned people. It is well established that nonmelanoma skin cancer is almost entirely caused by chronic exposure to solar ultraviolet radiation (uvr), and malignant melanoma is strongly related to ultraviolet radiation exposure. In the U.S.A., 300,OOG400,000 new skin cancers develop per year, almost all of which are preventable. About 5,000 deaths per year are due to nonmelanoma skin cancer, and a similar number are due to malignant melanoma. Reducing outdoor exposure during the 2 hr between 11 AM and 1 PM will prevent 50% of the daily carcinogenic uvr dose from reaching the skin. Most of the skin cancers occur in a small proportion of the population (particularly Irish, Scottish, and Welsh) who are genetically predisposed to sunburning and who suntan little. Anthropogenic disturbance of the stratospheric ozone layer by aircraft exhaust and chlorofluorocarbons may result in a significant increase in solar uvr reaching earth and may cause an increase of 60% or more in skin cancer incidence early in the next century.

Examination of the sun’s role in the production of human skin cancers does not lend itself to direct experimentation. However, extensive astute observations have strongly suggested the etiologic significance of light energy in the induction of these tumors. Skin cancers in Caucasians, in general, are most prevalent in geographic areas with the greatest insolation and among people who receive the most exposure, i.e., men who work outdoors. They are rare in Blacks and other deeply pigmented persons who have the greatest protection against ultraviolet radiation (uvr) injury. Further, the lightest-complexioned persons, such as those of Scottish and Irish descent, appear to be most susceptible to skin cancer formation when they live in geographic areas of high ultraviolet exposure. When skin cancers do occur in the darkly pigmented races, they are not primarily distributed in the sun-exposed areas as they are in light-skinned people. The tumors in these pigmented individuals are more commonly stimulated by other forms of trauma, ’ Presented at the Amertcan Health FoundationiDeutsche Prevention of Cancer: Assessment of Risk Factors and Future

Krehshilfe Directions,

Conference N.Y., N.Y.,

on the Primary June 7-8, 1979.

227 009 I-7435/8OlO20227-04$02.00/O Copyright 8 1980 by Academic Press, IDE. All rights of reproduction in any form reserved

228

FREDERICK

URBACH

such as chronic leg ulcers, irritation due to the lack of wearing shoes, the use of a Kangri (an earthenware pot that is filled with burning charcoal and strapped to the abdomen for warmth), the wearing of a Dhoti (loin cloth), etc. In contrast, the distribution of skin cancer in the Bantu albino and in patients with xeroderma pigmentosum follows sun-exposure patterns. INCIDENCE

OF SKIN CANCER IN MAN

Surveys of the incidence of skin cancer have been performed with varying success in the recent past. The major surveys have been those performed by the U.S. National Cancer Institute (“Ten City Survey”) (l), the recent Third National Cancer Survey (1971-72) (6) and the M. D. Anderson Institute, Texas Survey (1962-72) (5). Data are also available from Iowa (3), Minnesota (4), and several prevalence studies carried out in Australia (2). The enumeration of skin cancer incidence in a population is made very difficult by the relative benignity of the disease, which allows for curative treatment in physicians’ offices rather than in hospitals. Surveying all inhabitants in any one area is difficult and expensive, so that most skin cancer incidence studies have seriously underestimated actual conditions. The Third National Cancer Survey shows that the annual incidence rates for skin cancer (excluding malignant melanomas) vary from 539/100,000 in Dallas-Fort Worth to 174/100,000 in Iowa, in men, and 259/100,000 in Dallas to 83/100,000 in Iowa, in women. Skin cancer, like most other forms of cancer, is a disease that occurs more frequently in older persons. However, unlike other types of cancer, the rates for skin cancer are significantly higher even in the middle-aged persons, i.e., 35-54 years. Comparing the older and more recent studies strongly suggests that skin cancer has increased in the past several decades. For instance, comparing the metropolitan areas of Minnesota, an apparent twofold increase has occurred since 1963, perhaps a reflection of the recent trend for many people to expose themselves more, and more of themselves, to the sun’s rays as lifestyles, clothing, and leisure time have changed. The best estimate for present annual incidence of nonmelanoma skin cancer in the U.S. (in Caucasians) is 165/100,000 population. This means that at present about 300,000 cases of skin cancer develop in the U.S. each year, or about onethird to one-half of all cancers of all sites arise in the skin. Of this total a very small fraction is known to be caused by contact with PAH carcinogens (primarily cancer of the skin of the scrotum) and by exposure to inorganic arsenic or ionizing radiation. SUNLIGHT

AND THE ETIOLOGY

OF MALIGNANT

MELANOMA

The influence of latitude of residence on the incidence of and mortality from melanoma is the original and strongest evidence of importance of exposure to sunlight by white people as a cause of malignant melanoma. The gradient with latitude of death rate from malignant melanoma is not as great as for the other skin cancers, but it is substantial. Both the incidence and the mortality rates from malignant melanoma are rising rapidly in all countries in which they have been

CONFERENCE:

PRIMARY

PREVENTION

OF CANCER

229

studied. Mortality rates are rising by about 3% per year, and the incidence rates have more t.han doubled in the last 10 years. In some countries, e.g., Canada, the rate of increase is greater than that of any other tumor except lung cancer in men. These changes have been shown to be independent of improved diagnosis or certification. All these relationships are most unlikely to be due to chance. Where exposure of particular sites is different between the sexes because of conventional dress and hair styles (ears and neck in men; lower limbs in women), the exposed site has higher incidence and mortality rates than the unexposed sites in the opposite sex. While no substantial study of the influence of occupation has apparently yet been made, patients with malignant melanoma have higher numbers of hours of exposure and were less pigmented than controls, but at the same time seem to be concentrated in the more affluent strata of society. The present annual incidence of melanoma of the skin in the U.S. is approximately 6.4/100,000 in men and 5.9/100,000 in women or about 12,000 new cases per year. RELATIONSHIP

BETWEEN

SOLAR EXPOSURE

AND SKIN CANCER

There is extensive evidence that skin cancer, at least of the nonmelanoma type, is primarily caused by cumulative exposure to uvb radiation. This is true of skin unprotected by significant amounts of melanin, and thus mainly applicable to the white races, or about one-third of the world’s population. The cumulative dosage or exposure to uv radiation in sunlight is a function of the amount of ozone in the stratosphere, atmospheric conditions (cloudiness, extent of damage from aerosols, etc.), latitude, and lifestyle (which includes time and type of outdoor activity). Of these, the thickness of the stratospheric ozone layer is the major determinant of the amount and spectral distribution of effective uv radiation that can reach any point on earth. Approximately 60% of the day’s total carcinogenic radiation is received between 10 AM and 2 PM (local standard time). The relative effect of avoiding sunlight during this time is significantly large. Even for an office worker, whose small exposure occurs primarily on weekends and vacations, the reduction is more than 25% by avoiding just 1 hr from 12 noon to 1 PM. By avoiding two noontime hours, all occupations can achieve reductions in uv exposure of 35 to 50%. Thus, independent of location, by using a simple plan of avoiding sunlight during the noontime period, an individual can reduce exposure substantially without major changes in living habits. In addition to solar exposure, the effect of solar ultraviolet radiation is confounded by a host of variables such as ability to tan, age, complexion, eye color, hair color, etc., most of which are the result of genetic background. In contrast to nonmelanoma skin cancer, which is certainly related to cumulative effects of exposure to solar ultraviolet radiation, melanoma skin cancer seems also to be related to ultraviolet radiation, but by a different, not yet determined, mechanism which could, in part, depend on interaction with environmental chemicals. EFFECTS OF REDUCTION OF STRATOSPHERIC INCIDENCE OF SKIN CANCER

OZONE ON THE

The striking effect that personal characteristics have on the risk of developing nonmelanoma skin cancer from solar ultraviolet radiation makes it very difficult to

230

FREDERICK URBACH

reach realistic conclusions on the possible effect of changes in stratospheric ozone thickness on possible future changes in the incidence of skin cancer. Using the figures for incidence of skin cancer in the U.S. obtained by the Third National Cancer Survey, and a variety of assumptions on the relationship of uv radiation and skin cancer, several models for the potential effect of reduction of the stratospheric ozone layer have been proposed. Underlying all of these models are several assumptions: (a) a decrease in concentration of stratospheric ozone will result in an increase in uv radiation shorter than approximately 320 nm, and (b) the observed increase in skin cancer with decreasing latitude is due to several interacting factors, of which ozone thickness is one, and a variety of differences in local atmospheric conditions, genetic background of the population, and type, length, and kind of outdoor exposure. Other, not yet specified conditions, make up the rest. The various models attempting to relate decrease in atmospheric ozone to a possible increase in skin cancer result in the following “amplification” factors: (a) increase in uvr due to reduction in ozone concentration in the stratosphere: “amplification” factor 2x (range 1.4 to 2.5); (b) increase in skin cancer incidence due to increase in uv radiation at ground level: “amplification” factor 2x (range 0.5 to 5); and (c) increase in skin cancer incidence due to reduction in ozone concentration in the stratosphere: “amplification” factor 4x (range 0.7 to 12.5). The best estimate at present for reduction of the stratospheric ozone layer by continued emission of chlorofluorocarbon at 1979 rates is 15% at equilibrium. Considering the “amplification” factors described above, this could result in a 60% increase in the incidence of nonmelanoma skin cancer early in the next century. REFERENCES 1. Dom, H. F. Illness from cancer in the United States. U.S. Public HCUIIII Rep. 33, 59, 65, 77 (1944). 2. Gordon, D., Silverstone, H., and Smithhurst, B. A. The epidemiology of skin cancer in Australia, in “Melanoma and Skin Cancer” (W. H. McCarthy, Ed.). New South Wales Government Printer, Sydney, Australia, 1972. 3. Haenszel, W., Marcus, S. C., and Zimmerman, E. G. “Cancer Morbidity in Urban and Rural Areas.” Public Health Service Publication 462, U.S. Government Printing Offtce, Washington, D.C., 1956. 4. Lynch, F. W., Seidman, H., and Hammond, E. C. Incidence of cutaneous cancer in Minnesota. Cancer 25, 83 (1970). 5. McDonald, E. J., and Heinze, E. B. “Epidemiology of Cancer in Texas.” Raven Press,New York, 1978. 6. Scotto, J., Kopf, A. W., and Urbach, F. Non-melanoma skin cancer among Caucasians in four areas of the United States. Cancer 34, 1333 (1974).