UV Radiation, Latitude, and Melanoma in US Hispanics and Blacks

Background  Little information exists on the epidemiology of melanoma and the role of solar radiation in the development of melanoma in pigmented populations.

Objective  To evaluate the relationship between exposure to solar radiation and the incidence of melanoma in US Hispanics and blacks.

Design  Population-based ecological study.

Setting  State cancer registries of New York, New Jersey, Illinois, California, Texas, and Florida.

Subjects  Patients with invasive melanoma recorded by cancer registries.

Main Outcome Measures  We obtained age-adjusted, race/ethnicity- and sex-specific incidence rates of melanoma from similar time periods from the 6 cancer registries.The incidence rate s were correlated with the annual mean UV index and the latitude of residency.

Results  For both Hispanics and blacks, the incidence of melanoma was positively associated with the UV index and negatively associated with the latitude ofresidency. Statistically significant correlation between melanoma and theUV index (R = 0.93; P =.01) and latitude (R = −0.80; P = .05) was observed in black males. Hispanics and blacks have a significantly lower incidence of melanoma than whites, with blacks having the lowest ratesof melanoma.

Conclusions  Exposure to solar radiation appears to play a role in the developmentof melanoma in both Hispanics and blacks. Sun protection and melanoma riskeducation should be performed in these populations.

The incidence of skin cancers shows substantial variation among populations of different racial composition and geographic location.¹ Amongdifferent racial groups, light-skinned populations have the highest rate of melanoma and nonmelanoma skin cancers, ie, basal cell and squamous cell carcinoma.The incidence of melanoma has been increasing faster than that of nearly every other cancer.^1,2 Therefore, muchattention has focused on the epidemiology and risk factors of melanoma inwhite populations.

A host of epidemiological evidence suggests that exposure to UV radiation (UVR) through sunlight is the major environmental risk factor for melanoma.^3-5 Both constitutive factors,such as natural protection against UVR exposure, and behavioral factors, such as the amount of UVR exposure, are important modifiers in the risk of developingmelanoma. Individuals with lighter natural skin color and greater tendency to burn (than tan) after sun exposure are at greater risk for melanoma.⁴ With regard to the amount of UVR exposure, epidemiologicalstudies have demonstrated a negative correlation between latitude of residence and incidence and mortality rates of melanoma in homogeneous, light-skinnedpopulations.^3,5,6 Migrant populations that move closer to the equator develop melanoma at rates higherthan in their country of origin, but lower than the rates prevailing among the native residents of their adopted country.^7-9

Little is known about the epidemiology and risk factors of melanomain Hispanics and blacks.¹⁰ The lack of studiesin these populations partly reflects the fact that Hispanics and blacks, both having darker skin pigmentation than whites, are at lower risk for developingskin cancers.^11-18 The data on cancer among Hispanics are limited because of the difficulty of classifyingHispanic race/ethnicity. Although the incidence of melanoma is lower in Hispanics and blacks, melanomas in these populations are more likely to metastasize,and carry a poorer outcome than in whites.^11,18-20 The worse outcome has been attributed to a higher proportion of acral lentiginousmelanoma (ALM) and delayed presentation to care in Hispanics^10,11,18 and blacks.^19,21-23 Itis not clear whether UVR exposure plays a similar role in these cancers in darker-pigmented populations.

From the public health and cancer control perspective, it is important to gain a better understanding of the epidemiological patterns of melanomain minority groups. Hispanics are among the fastest-growing populations in the United States . In 2002, approximately 37.4 million Hispanics composed13.3% of the total US population.²⁴ The Hispanic population is projected to reach 17% of the total US population by 2020.²⁵ As the population of Hispanics continues to increase,their cancer experience will have a substantially greater impact on cancer prevention and public health promotion in the United States.

We extracted melanoma incidence data from population-based state cancer registries and compared cancer rates among different race/ethnicity groups.We also analyzed melanoma rates in relation to estimated annual UV index andlatitude of residency.

We selected US states with large Hispanic populations and readily accessible cancer data. California, Florida, Illinois, New Jersey, New York, and Texasmet our selection criteria. According to the US Census Bureau 2000 reports, these states are among the 7 states (along with Arizona) with a populationof more than 1 million Hispanics (Table 1).²⁶ The populations in the 6 statesrepresent 73% of the total Hispanic population and 77% of the total black population in the United States. Also, the cancer registries in these statesare deemed by the National Program of Cancer Registries (NPCR) to have high-quality data collection; more than 95% of melanoma cases in all 6 registries are confirmedby microscopy.²⁷ These registries use Surveillance, Epidemiology, and End Results and North American Association of Central CancerRegistries standards and International Classification of Diseases for Oncology, Second Edition, codes for coding data and groupingof cancer sites. These states also represent the 4 distinctive regions of the United States: Northeast, Midwest, South, and West, therefore permittingthe evaluation of the relationship between geographic location and cancer incidence. We extracted age-adjusted incidence rates to allow comparison ofcancer incidence over time and across geographic regions and population subgroups, even when the age distributions were not comparable. The rates were adjustedto the 2000 US standard population. Data sets with the greatest similarity in time periods were selected for correlation analyses against UVR exposure.A total of 64 305 melanomas were included in our study. Cumulative cases of new melanomas by race/ethnicity and sex documented in each one of the cancerregistries for specific time periods are presented in Table 2.^28-33

We used annual mean UV index and latitude data for each state as surrogate estimates of UVR exposure to facilitate the evaluation of cancer incidencesin relation to UVR exposure and geographic locations. Geographic latitude of residence is a routinely used proxy measure for UVR exposure. The averagenorth-to-south latitude of each state was obtained and used as the representative latitude. The UV index is a composite measure of the expected risk of overexposureto UVR from the sun, computed by the National Weather Service on a next-day basis for 58 cities. After local conditions are accounted for, UVR at differentwavelength are weighted so that the wavelengths more harmful to human skin have higher weights. The final value is converted to an index number thatranges from 0, where there is no sunlight, to the midteens. For this study, we calculated the annual mean UV index of the major cities in the 6 studystates from 1997, the only year for which the monthly mean UV index was available.³⁴ The annual mean UV index was the sum of monthly means; in the states with more than 1 city, the average of all cities was used. Table 1 summarizes the mean annual UV indexand latitude of each state.

The standardized sex- and race/ethnicity-specific incidence data for melanoma were summarized with 95% confidence intervals. Pearson correlationanalyses were performed to examine the correlation between the incidence rateof melanoma and the UV index or latitude of the state of residency. The levelof significance was set at P = .05, 2-sided, for all statistical analyses.

We obtained available cancer incidence data that included Hispanic ethnicity from the 6 state cancer registries. Only cases of invasive melanoma were calculatedin the cancer rates. All 6 cancer registries determine Hispanic or Latino origin based on surname and maiden name in addition to self-reporting on medicalrecord and death certificate.^29,33,35,36 The protocol to impute Hispanic ethnicity from surname and maiden name varieswithin registries. The grouping of Hispanic or non-Hispanic population also differs. For instance, Hispanic, white, and black are 3 mutually exclusivepopulations in California and Texas.^33,37 In New Jersey, New York, Illinois, and Florida, Hispanic isan ethnicity variable; therefore, cancer data for Hispanics may overlap with those of whites and blacks.

Standardized incidence rates by race/ethnicity and state of residency, along with 95% confidence intervals, are presented for females in Figure 1 and for males in Figure 2. For both sexes, melanoma rates were the highest amongwhites, lower among Hispanics, and lowest among blacks. This trend was observed for rates within each state and across the 6 states. As expected, among whitefemales in each state, the incidence of melanoma was 4- to 6-fold and 7- to 18-fold higher than among Hispanic and black females, respectively. Amongwhite males in each state, melanoma occurred 4 to 7 times more frequently than among Hispanics and 14 to 29 times more frequently than among black males.During 1995 to 1999, whites residing in California had the highest overall melanoma rate among both sexes: 18.2 per 100 000 females and 28.8 per100 000 males. Hispanic females in California had the highest melanoma rate (4.2 per 100 000 in 1995-1999) among female Hispanics, while Hispanicmales in New Jersey had the highest rate (5.6 per 100 000 in 1996-2000) among Hispanic males. In black males, the highest rates were observed in Californiaand Florida (1.3 per 100 000 for 1995-1999 and 1.3 per 100 000 for1989-1995, respectively). Florida also had the highest melanoma rate for blackfemales (1.6 per 100 000).

The mean annual UV index and latitude of residency were correlated withthe incidence of melanoma in all race/ethnicity groups for both sexes (Table 3). The correlation coefficient betweenthe incidence of melanoma and the UV index was 0.56 (P = .24) in Hispanic females and 0.41 (P = .42) in Hispanicmales, but this did not reach statistical significance. There was a significant positive correlation between the incidence of melanoma and the annual meanUV index in black males (correlation coefficient, 0.93; P = .01). The melanoma incidence rates were negatively associated withthe latitude of residency state, meaning that the lower the latitude, thehigher the melanoma rates in all race/ethnic groups and in both sexes. The negative correlation was statistically significant in black males (correlationcoefficient, −0.80; P = .05).

This is the first study to use large population–based data to examine the relationship between melanoma and UVR exposure in Hispanics andblacks in the United States. We confirmed the association between melanoma, race, and UVR exposure. We verified that the incidence rates of melanoma arethe highest among whites, which confirms the protective role of constitutive pigmentation in the development of melanoma.

Variations in melanoma incidence within Hispanic and black populations in relation to UVR exposure and latitude also suggest that risk in these populationsis modified by behavioral patterns, unknown cultural-environmental exposures, and socioeconomic factors, as well as constitutive characteristics. Studieshave shown that severe sunburn or strong intermittent exposure, especially during childhood, is a high-risk exposure pattern.^38,39 Adultrecreational or intermittent sun exposure is also a risk factor for melanoma.^40,41 However, these conclusions are based on studies in white populations; it is not clear whether intermittent exposureto UVR holds similar importance in Hispanics and blacks. Nonetheless, the lower rates of melanoma in Hispanics and blacks may be influenced, in part,by behavioral characteristics. Evidence suggests that both Hispanics and blacksin the United States have less high-risk sun behavior than non-Hispanic whites.⁴² Hispanics and blacks may also have a higher levelof chronic sun exposure through occupation than whites. According to the USCensus, Hispanics²⁴ and blacks⁴³ werealmost twice as likely to work as operators and laborers than non-Hispanicwhites. Conversely, a greater proportion of non-Hispanic whites held managerialor professional occupations than Hispanics or blacks. Several studies have found that chronic and occupational exposure to sunlight paradoxically lowersthe risk for melanoma.^41,44

The role of solar radiation in melanoma among nonwhites has not been elucidated. Based on previous reports of increased proportion of ALM occurringin Hispanics and blacks,^11,45,46 direct sun exposure was thought not to be an important etiologic factor for melanomain these populations because ALM behaves differently from the superficial spreading type commonly seen in whites and because ALM occurs more often onbody areas that are protected from the sun.^23,47,48 We were not able to examine the role of UVR exposure specifically in the developmentof ALM, as incidence rates by melanoma subtype were not available. Nonetheless, we found that the rate of melanoma in both Hispanics and blacks increasedwith increasing annual UV index or lower latitude of residency. These results suggest that UVR exposure may have a greater than expected role in the developmentof melanoma in darker pigmented populations. We also found that the degree of the importance of UVR exposure in different races may vary within populations.While the association of melanoma with UVR exposure was seen in whites, blacks, and Hispanics, the association reached statistical significance only in blackmales (P = .01 for melanoma incidence and UV index; P = .05 for melanoma incidence and latitude). Our study is the first to demonstrate such a correlation between the incidence of melanomaand UVR exposure in blacks. Interesting, one recent population-based study found that the mortality rates of melanoma in black males increased significantlywith increasing levels of exposure to surface UV-B radiation; however, the authors did not report significant increases in melanoma incidence with UV-Birradiation in black males or females.⁴⁹

Several inherent limitations related to cancer data from population-basedregistries exist. First, the reliability of race- or ethnicity-specific ratesdepends on the accuracy of classification in cancer cases and in population estimates. Undercounts of both cancer cases and population may occur. Countsof melanoma cases in each registry likely underestimate the actual rates in the population because a portion of the cases that are diagnosed and treatedon an outpatient basis are not reported to the registries. Inconsistencies and difficulties associated with defining Hispanic ethnicity suggest a generaltendency for undercount of Hispanic populations. Information available from state cancer registries was limited to counts and incidence rates. Therefore,we did not perform correlation studies by tumor thickness or by melanoma subtype, such as ALM vs superficial spreading type.

We used melanoma data from 6 states in the United States. Although these states are representative of the regions in the United States and are madeup of more than 70% of Hispanic and black populations in the United States, evaluation of wider geographic areas will provide a more accurate understandingof the epidemiological pattern of melanoma. Incidence rates in some registries differed in time periods, and consistent data from registries for a commontime period were not available on Hispanics. Nevertheless, we believe that our results are valid, as the time periods are all within the last decadeand the melanoma rates in Hispanics and blacks have been relatively stable.^50,51

Also, the UV index from one or a few cities in a particular state was used to represent the actual UV index of that state, and was used as a surrogatemeasure of individual exposure and as such is at risk for ecological fallacy. We did not examine the individual sun exposure history or patterns in thestudy population; therefore, it is not known if or how these factors affected the strength of our analysis.

Differences in racial composition and migration patterns among the various subgroups of Hispanics are additional confounders in our studies. A personof Hispanic origin could be of any race or origin such as Mexican, Puerto Rican, Cuban, or Central or South American. For instance, Mexicans have moreindigenous American heritage than Cubans, who have more of African genetic heritage.⁵² Within the United States, Hispanicpopulations vary with geographic location.^24,52 The heterogeneity of constitutive skin pigmentation in the Hispanic populationscomplicates our analysis of melanoma rates against UVR exposure. Migration histories of the Hispanic populations are also unknown in our study. The majorityof Hispanics in the United States are second-generation immigrants.²⁴ The second generation of Hispanics likely have been adapting to the climatic conditions, cultural beliefs about sun exposure,and recreational practices in the United States.¹⁰ Whether Hispanics in the United States have acquired or modified their risk factorsis not known, but ongoing studies are assessing sun behaviors and attitudes among Hispanics.

Although both Hispanics and blacks have lower rates of melanoma thanwhites, the impact of melanoma is of paramount importance from a public healthperspective. It is predicted that the incidence of melanoma will continue to rise as a result of the continuing decrease in the concentration of stratosphericozone as well as of the increasing exposure time to sunlight through leisure sunbathing, both of which increase exposure to solar UVR.⁵³ Ourstudy suggests that UVR exposure plays a role in the incidence of melanomain darker-pigmented populations as well as in fair-skinned populations. Sinceimproved preventive measures and educational efforts have affected the diagnosis and outcome of melanoma cases, it is not unreasonable that similar publiceducation campaigns be directed at the nonwhite population as well.

Further studies, with a broader population base, that incorporate information on the racial/skin subtypes among Hispanics and blacks, with more accurateestimates of sun exposure and stratification of melanoma subtypes, are needed to elucidate the role of UVR exposure as an etiologic factor in melanoma riskin these populations. Such comparisons of melanoma trends among different populations can help to better define the relative contributions of constitutivefeatures, environmental exposures, and lifestyle to the development of melanoma.

Correspondence: Robert S. Kirsner, MD, Department of Dermatology, University of Miami School of Medicine, 1201 NW 16th St, Miami, FL 33125 (Rkirsner@med.miami.edu).

Accepted for publication October 8, 2003.

The Florida cancer incidence data used in this report were collected by the Florida Cancer Data System under contract with the Department of Health.

The views expressed herein are solely those of the authors and do not necessarily reflect those of the contractor or the Department of Health.