The assessment of dermoscopic images is based on the evaluation of color, shape and deep structures in the skin moles. Then image quality is of paramount importance for the diagnosis accuracy.
Incorrect color information of the image, its under- or over-exposition, the lack of sharpness and focus clarity, a low spatial resolution have a negative impact and lead to misdiagnosis. The importance of color[1]
Human skin, especially the epidermis, contains several chromophores and among them hemoglobin and melanin. They strongly absorb light in the ultraviolet (UV) and visible ranges. Hemoglobin is found in the microvascular network of the dermis, typically 50–500 μm below the skin surface. In contrast, melanin is located in the epidermis, which occupies the top 50–100 μm; it is present in human and animal skin to varying degrees, and is responsible for unique eye, hair and skin color. The substance also absorbs harmful UV (ultraviolet) rays and protects the cells from sun damage.
Melanin, produced in melanocytic cells, is the most important chromophore in pigmented lesions. When melanin is located in the stratum corneum or the upper epidermis appears as black on dermoscopy; when it is located in the mid or low epidermis it appears as brown, and it appears blue-gray when it is present in the dermis. The knowledge of colors on dermoscopy is relevant since it allows to identify the deepest component of a melanocytic tumor (blue area) and accurate color depiction of the mole and surrounding skin is important in the diagnostic assessment.
A skin cancer is the abnormal growth of skin cells. There are three major types of skin cancer — basal cell carcinoma, squamous cell carcinoma and melanoma.
Melanoma develops when melanocytes start to grow out of control; it is much less common but is more dangerous because it is much more likely to spread to other parts of the body if not caught and treated early.
Color information is used for distinguishing benign and malignant melanocytic lesions: benign lesions tend to exhibit one or two colors, whereas malignant lesions exhibit more colors. Most of malignant melanomas are characterized by the presence of six suspicious colors that from a histopathologic perspective indicates the presence of melanin in the deeper layers of the epidermis. A study[2] evaluated the role of color features (a set of fifteen features) in malignancy detection.
In melanoma even a hard-to-detect hue difference could make a great difference in diagnosis[3].
Other important determinants of lesion color include hemoglobin in red blood cells (pink to red colors), lipids or keratin (yellow) and collagen fibers in the dermis (white color).
Some colors, especially when associated with some dermoscopic structures, have important histopathologic correlates[4]:
- white globules (associated with balloon-cell changes) (Jaimes et al.),
- homogeneous yellow to orange areas (associated with cell xanthomization) (Bañuls et al.)
- black lacunae (associated with blood thrombosis).
The importance of color reproduction in dermoscopy analysis will be continued in next blog post.
[1] See also https://dermoscopedia.org/, the education portal coordinated by the International Dermoscopy Society (IDS)
[2] S. Pathan et al - Melanoma Detection in Dermoscopic Images using Color Features - Biomedical & Pharmacology Journal, March 2019 - Vol. 12(1), p. 107-115
[3] C. Pagliarello , I. Stanganelli et al. - Digital Dermoscopy Monitoring: Is it Time to Define a Quality Standard? - Acta Derm Venereol 2017; 97: 864–865
[4] Jaimes et al.: White globules correlate with balloon cell nevi nests. J. Am. Acad. Dermatol. 2011;65:e119-e120.
Bañuls et al.: Yellow and orange in cutaneous lesions: clinical and dermoscopic data. J Eur Acad Dermatol Venereol 2015;29:2317-25.