1 Radiation Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD.
2 Biostatistics Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD.
3 Department of Dermatology, Bufalini Hospital, Cesena, Italy.
4 Genetic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD.
Received for publication December 3, 2001; accepted for publication April 17, 2002.
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ABSTRACT |
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case-control studies; colorimetry; epidemiologic methods; melanoma; risk factors; skin pigmentation; spectrophotometry, ultraviolet; ultraviolet rays
Abbreviations: Abbreviations: CIELAB, Commission International dEclairge 1976 L*a*b* [colorimetric system]; CMM, cutaneous malignant melanoma; MED, minimal erythema dose.
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INTRODUCTION |
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Well-established CMM risk factors include fair skin, red or blond hair, blue or green eyes, propensity to burn, inability to tan, and, to a lesser extent, both intermittent and cumulative sun exposure (1317). Substantial evidence on the association of these risk factors with CMM comes from case-control studies that used retrospective assessment of sun exposure and self-reported information on individual sensitivity to ultraviolet radiation (16). This approach might result in imprecise assessment of exposures and potentially differential reporting between cases and controls. Indeed, sun sensitivity, as assessed by ability to tan but not by hair color, was found to be subject to recall bias in one study (18). However, another study found that reporting of tanning and burning was less biased than reporting of sunbathing in childhood and adulthood, and possibly reporting of freckling in childhood (19). Accurate measurements of pigmentary traits and skin reaction to sun exposure are necessary to obtain more precise estimates of CMM risk and to adjust for other risk factors in analyses.
One instrument used for the objective measurement of constitutive skin color is the portable tristimulus reflectance colorimeter (2023). Under standardized conditions, when distortion of instrument readings is minimal, this method shows high reproducibility (21) and correlation with measurements based on direct ascertainment of pigmentary chromophores (20, 24). The instrumental method of evaluating cutaneous sensitivity to ultraviolet radiation is based on measurement of the minimal erythema dose (MED)the minimal dose of ultraviolet radiation able to provoke perceptible erythema of the skin 2024 hours after exposure (25, 26). Although not a perfect measure itself (27, 28), MED provides a direct and quantitative measure of the skins sensitivity to ultraviolet light. Constitutive skin color and skin sensitivity to ultraviolet radiation have been extensively studied in healthy volunteers or groups of patients with different skin diseases (25, 27, 2932) and in a few well-defined epidemiologic studies (3338). However, earlier studies did not use colorimeters or specifically analyze hue and chroma values.
We report here the results of a case-control study of melanoma conducted in Cesena, Italy. The main focus of this paper is on the association between melanoma and instrumental measurements of skin color and ultraviolet light sensitivity. In addition, we investigated the relation between instrumental and questionnaire-based data and the modifying effect of sun exposure on CMM risk associated with instrumental measurements.
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MATERIALS AND METHODS |
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Data collection
Interview and skin examination
Trained interviewers who were blinded with respect to case/control status administered a questionnaire to all subjects. Lifetime residential history of at least 6 months occupancy was recorded. The interviewers obtained information on sun exposure history (particularly between the hours of 11:00 a.m. and 3:00 p.m.), frequency and duration of sports and outdoor activities by age, and types of vacations taken. In addition, information on frequency and duration of occupational exposure to sunlight, use of ultraviolet sunlamps and sunscreens, and history of sunburns was recorded. For assessment of sunlight sensitivity, subjects were asked how their skin would respond to an initial 30 minutes of direct strong sunlight and how it would respond after repeated and prolonged exposure to sunlight. Information on sociodemographic characteristics, medical and family history of cancer and other diseases, smoking history, alcohol consumption, and diet was collected.
In addition, one dermatologist performed skin examinations of the entire body, except the genital area, for all subjects. Skin color was evaluated on the inner forearm and characterized on a three-category scale as dark/olive, medium, or light; eye color was characterized on a nine-category scale as black, dark brown, light brown, brown-green, green, blue-green, dark blue, light blue, or gray; and natural hair color was characterized on a six-category scale as black, dark brown, light brown, reddish-brown, blond, or red (40). Multiple lightly pigmented macular lesions commonly present on the face, upper back, and arms were defined as freckles. Diagnosis of dysplastic nevus and number of nevi were based on a review of photographs of the subjects back by an oncologist who was blinded as to case/control status (40). To be counted as a dysplastic nevus, a nevus had to be 5 mm or greater in diameter, be predominantly flat, and have at least two of the following criteria: variable pigmentation, indistinct borders, and an irregular outline (41). Nevi were defined as pigmented macules or papules greater than 2 mm in diameter that did not include freckles, lentigines, keratoses, and other pigmented lesions.
Measurement of skin color
Skin color was measured on the buttock with a Minolta CR-300 colorimeter (Minolta Company Ltd., Osaka, Japan). This instrument is a reflectance spectrophotometer that measures reflected light in the visible spectrum (range, 400700 nm). It also works as a tristimulus chromameter, recording colors in a three-dimensional space known as Commission International dEclairge 1976 L*a*b* (CIELAB) color space (20, 21). Every color in the CIELAB colorimetric system can be described by a combination of three coordinates, L*, a*, and b* (42, 43), where L* is the total quantity of light reflected or skin brightness (described as light, dark, etc.), a* represents color ranging from red (positive values) to green (negative values), and b* represents color ranging from blue (negative values) to yellow (positive values). The a* and b* coordinates can be converted into polar coordinates (21, 43) defined as hue angle (h° = arctan(b*/a*)) and chroma, often referred to as saturation of color (C = [(a*)2 + (b*)2]1/2). Hue refers to the basic color of an object, where 0° represents red and 90° represents yellow. The hue angle in our population ranged from 43° to 82°. Chroma describes the intensity of color, with higher chroma indicating greater intensity.
The instrument was white-calibrated and turned on at least 15 minutes before the start of each set of measurements. Measurements were taken by a single dermatologist after 15 minutes acclimatization in an air-conditioned room at 1920°C. Prior to measurement, the subject was placed in a horizontal position to avoid orthostatic effects having an influence on skin color. During measurement, the measuring head was held steady, perpendicular to the skin surface. Special care was taken not to apply excessive pressure on the head of the instrument to avoid venous congestion that could artificially distort the measurements (21). Measurements of L*, a*, and b* were repeated three times consecutively and averaged. The a* and b* values were then reexpressed as hue and chroma values.
Measurement of MED
The sensitivity of skin to ultraviolet radiation was assessed by measurement of MED. Skin was irradiated with simulated solar radiation from an artificial ultraviolet light source (the IL1700 radiometer, Blue-Point model; International Light, Inc., Newburyport, Massachusetts) and measured by an International Light detector (SED240/SCS280/W, head model; International Light, Inc.) equipped with a cousin response diffuser. A spectroradiometer (SpectraMED model; Flyby s.r.l., Livorno, Italy) was used to characterize the spectral features of the ultraviolet lamp radiation, which extended from 290 nm to 440 nm (covering the entire solar spectrum of ultraviolet light reaching the earths surface), and to evaluate the erythemal effective irradiance at the surface of the skin, which was 1.65 mW/cm2. Depending on the subjects constitutive skin color, a suberythemal dose of ultraviolet light (on the order of 1020 mJ/cm2) and additional incremental doses comparable with biologic exposures to sunlight (44) were administered by a single nurse on a very small area of buttock skin. Each area of irradiation was circled with a fine-point waterproof permanent black marker. For skin types very sensitive to ultraviolet radiation, the dose ranged between 15 mJ/cm2 and 67 mJ/cm2, while for individuals with darker skin, the dose ranged between 21 mJ/cm2 and 108 mJ/cm2. Incremental doses of ultraviolet radiation were as follows: 15/21, 27, 40, 54, 67, and 94/108 mJ/cm2. Subjects returned to the hospital the following day so that the marked skin areas could be checked. The MED was defined as the lowest dose of radiation that produced the minimum noticeable (by the nurses visual inspection) reddening of the skin 24 hours after exposure. This approach has been used and validated multiple times in subjects from the same area.
Statistical analysis
Nonparametric correlation analysis and one-way analysis of variance were performed to explore the relation between instrumental measurements of skin color and skin sensitivity and other characteristics among control subjects. The odds ratio was used as the measure of association between potential risk factors and CMM. Unconditional logistic regression models (45) were fitted to estimate odds ratios and compute 95 percent confidence intervals. Likelihood ratio tests (two-sided tests with an level of 0.05) were used to test hypotheses. All analyses were adjusted for matching factors (age at interview and sex). In addition, potentially confounding factors were taken into account where specified. We repeated all analyses excluding the hospital volunteers or including spouses/partners of the cases as the only control group, and we did not find major differences. This paper reports results obtained using the entire control group.
The associations between CMM and L*, hue, chroma, and MED are also presented in terms of the exponential increase in the odds ratio per unit of measurement. These estimates were derived from the logistic regression model using the continuous measurement variables. Because of the larger range of values for the MED measurement, we present the exponential increase per 10 mJ/cm2.
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RESULTS |
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Table 3 shows odds ratios for CMM associated with instrumental measurements (both categorical and continuous) of skin color and skin sensitivity after adjustment for matching factors (age at interview and sex). Odds ratios for CMM increased significantly with increasing levels of L* and decreasing levels of chroma and MED, although the latter trend was not monotonic. Odds ratios for hue angle were elevated but not significant. Prior analysis demonstrated that in this population there was no association between CMM risk and lifetime hours of sun exposure between 11:00 a.m. and 3:00 p.m. from either recreational activity or occupational activity (40). In addition, inclusion of sun exposure covariates in the model did not change risk estimates for any of the instrumental measurements (data not shown).
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DISCUSSION |
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Traditionally, epidemiologic studies have relied either on self-assessment of skin color or its assessment by an observer against referent color standards. In both cases, the resulting color is a subjective perception that often fails to distinguish between the pigmented substances in the skin and the actual sensation of color. In an effort to improve on these subjective measures, some researchers have attempted to measure melanin content objectively by studying skin color reflectance after irradiation with wavelengths similar to those of ultraviolet light (35, 37, 38). The present study used a portable tristimulus reflectance spectrophotometer that assesses skin color and its main components, brightness, hue, and chroma, in a manner analogous to the human eye (20, 21, 23, 43). While such measurements are objective and more accurate, they still do not provide direct information about the primary chromophore responsible for color (23).
L* measures the brightness component of color, and it varies on an achromatic gray scale between a value of 100 for a white surface and 0 for a black surface. Previous studies have shown that L* is negatively correlated with the amount of epidermal melanin, showing an approximately linear relation at low melanin levels (20, 46). Colorimetric measurements appear to be affected by the amount of hemoglobin in the superficial vascular plexus (20, 46) and skin surface structure (21, 23). In addition, a high serum ß-carotene level may affect skin color (47) and consequently colorimetric estimate. Although our study included subjects of Italian origin, L* measurements taken on the buttock met criteria for low pigmentation (L* > 60) (46). As might be expected on the basis of the relation between L* and amount of melanin, higher L* levels were associated with lighter skin, increased sensitivity to burning, and reduced tanning ability, as well as lower actual sun exposure from recreational activities. This latter finding suggests active sun avoidance by subjects with lighter skin. Similarly to other investigators (48), we did not see variation in L* values by eye color or hair color. CMM risk associated with L* was consistently increased in all of the analyses and persisted after adjustment for a variety of potential confounders. Interestingly, when skin color and L* were included in the same model, CMM risk estimates for both factors, while attenuated slightly, were not materially changed. This might imply that skin color and L* carry additional information not accounted for by the other factor.
The chroma component of color or saturation, which is visually perceived as "weak" or "strong," does not have a clear physiologic determinant. We observed a strong correlation between L* and chroma, while b* and chroma were correlated by construction. Allowing that b* value is a good indicator of tanning (24, 49) and that our findings show similar variation in L* and chroma by phenotypic and sun-related characteristics among controls, it appears that L* and chroma are both related to skin pigmentation. In univariate analyses, chroma was associated with increased risk of CMM. However, after we included L* and chroma in a single model, the effect of chroma disappeared while the effect of L* remained unchanged. This pattern suggests that in our data, either L* is the main risk factor for melanoma or both factors are related to CMM risk but chroma is not an independent predictor of risk.
The hue component of color, another chromaticity coordinate, was positively correlated with L*. However, unlike L* or chroma, it did not vary significantly by skin color or skin reaction to sun exposure and was not associated with CMM risk. This implies that hue angle is not a direct indicator of skin pigmentation or other skin characteristics associated with CMM risk.
MED is a well-accepted and often preferred parameter for evaluation of a subjects sensitivity to phototherapy or solar radiation (26, 28, 5053). However, it has several limitations (27). Since MED determination is made by humans, it may be subjective, and interobserver variability could take place. In addition, MED values are limited to the specific categories of ultraviolet doses used, and uncontrolled unknown factors might influence the degree of individual vascular dilation. Moreover, it requires that subjects make two clinic visits, one for irradiation and the second for evaluation of erythema after 24 hours. This may affect participation rates and the complexity of field activities. Although we cannot exclude the effect of uncontrolled factors, it is unlikely that MED measurements in this study were subject to systematic error, since a single trained nurse who was blinded with respect to the subjects status performed the readings.
Our study showed a negative association between MED and L*, which is consistent with most other studies (25, 5458). It is possible that melanin, because of its absorption spectrum, influences both primary defense against ultraviolet radiation and skin color (57). Indeed, MED significantly varied by category of constitutive skin color and, to a lesser extent, by reaction to acute and chronic sun exposure in our study. This may reflect the subjectivity of self-reported skin reaction to sunlight, which may not be a reliable measure of ultraviolet light sensitivity. Other studies using alternative methods to measure skin reaction to ultraviolet radiationincluding slope of the erythemal dose-response curve (29, 58, 59), ratio between facultative and constitutional skin color (5860), colorimetric quantification of erythemal response after irradiation (60), and measurement of minimal immediate pigment darkening dose and minimal delayed tanning dose (58)have reported variable associations with skin type. In our study, MED was associated with increased risk of CMM; this finding is in agreement with one study that found higher light sensitivity among melanoma patients (61) but not with other studies (55, 62). However, none of the previous studies conducted risk estimation or adjustment for potential confounders. In this study, after adjustment for hue and chroma but not for L*, risk estimates associated with MED remained elevated, providing supporting evidence that risk depends mainly on melanin pigmentation, with other characteristics of the skin, such as epidermal thickness, possibly playing a role.
The role of sun exposure in the etiology of CMM is unresolved (14, 17). It was not associated with increased risk of CMM in our study (40), even after we controlled for the presence of dysplastic nevi and pigmentation characteristics. Since most control subjects were spouses/partners of the cases, these groups had similar levels of adult sun exposure. In addition, controls lived in the same geographic areas as cases, most for their entire lives. Thus, differences in recreational sun exposure between cases and controls were not large. However, sun exposure could still be an important modifier of the effect of potential sensitivity to the sun on melanoma risk. We did not find any interaction between subjective measurements of pigmentation characteristics and sun exposure in the association with melanoma risk, but we observed an increased risk with L* and MED only among subjects with a high number of cumulative hours of sun exposure. Similar patterns were observed when recreational and occupational sun exposures were analyzed separately; however, the statistical power of these tests was lower. It is unlikely that our findings could be attributable to greater misclassification of L* among subjects with low total sun exposure, since results were similar for L* and MED and measurements were made on the buttock, an area that is not affected by sun exposure. In addition, these findings are in agreement with those of other case-control studies (6368) that measured sun exposure and sun sensitivity in a number of different ways. This suggests that skin sensitivity in the presence of sun exposure is an important risk factor for CMM and that reduction of sun exposure could be especially beneficial for ultraviolet-sensitive persons. Finally, this finding further supports the use of instrumental measurements in epidemiologic studies. Use of these instruments can help researchers to identify those elements of the pigmentary trait that relate to CMM risk. In addition, by adjusting for the risk due to pigmentation phenotype, investigators can more fully evaluate associations with other potential risk factors for melanoma, such as sun exposure, sunscreen use, and genetic determinants.
In our study, skin examination was performed by a single dermatologist. Participation rates were high for both cases and controls. Detailed information on a large number of risk factors was collected and analyzed. A possible weakness of the study was the heterogeneous nature of the control series, which included spouses/partners of cases, hospital controls, and healthy volunteers. However, limiting the analyses to spouses/partners did not materially influence our findings. While the measurements of skin color and sun sensitivity were within the range of those reported in studies conducted among populations of other European origins, generalization of these findings to fair-skinned populations should be cautious. Finally, the limited statistical power of these findings, especially for certain subgroup analyses, should be noted.
In summary, brightness of constitutive skin color and MED in the presence of sun exposure emerged as important risk factors for CMM in a case-control study conducted in an Italian population. This association was largely independent of other phenotypic and sun-related characteristics. Epidemiologic studies of melanoma and other skin diseases may benefit from the inclusion of instrumental measurements along with traditionally assessed risk factors to obtain more accurate, quantitative, and objective information.
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ACKNOWLEDGMENTS |
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The authors are indebted to Dr. G. Landi for overseeing the study procedures and to Angela Aguzzoni for MED assessment. The authors are grateful to Paola Minghetti for organizational and technical support.
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NOTES |
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REFERENCES |
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