Pump-Probe Imaging Differentiates Melanoma from Melanocytic Nevi

Multiphoton imaging reveals chemical changes in melanoma compared to benign nevi and could enhance current clinical diagnostic protocols. A Wolf in Sheep’s Clothing It is frequently difficult to distinguish whether something is dangerous or harmless. In the case of melanoma diagnosis, a misdiagnosed lesion could have deadly consequences. Rightly, doctors err on the side of caution; however, false-positive diagnoses result in unnecessary surgeries and biopsies, as well as emotional distress for the patient. Matthews et al. have developed a new imaging technique that can distinguish melanoma from benign lesions, which in concert with current techniques could improve patient diagnosis and decrease the need for unnecessary tests. The pigment melanin is the primary determinant of skin color. There are two dominant types of melanin in melanocytic lesions: eumelanin and pheomelanin. Eumelanin, which is brown/black, is the most common biological form of melanin, whereas pheomelanin is largely responsible for red hair and freckles. The authors use a multiphoton imaging technique, pump-probe spectroscopy, to determine the ratio of these different melanins in melanocytic lesions in the context of lesion architectural and cytological features. Eumelanin was found at higher levels in melanoma compared with both dysplastic and benign nevi. When combined with pathological examination, imaging-based determination of the melanin ratio decreased the number of false-positive diagnoses compared with pathological examination alone. Moreover, this imaging technique could be used on hematoxylin and eosin–stained slides, which are currently used by pathologists for melanoma diagnosis, and may even be able to be adapted for noninvasive diagnostics. In conjunction with traditional diagnostic methods, melanocytic imaging should greatly improve doctor’s ability to sort the wolves from the sheep. Melanoma diagnosis is clinically challenging: the accuracy of visual inspection by dermatologists is highly variable and heavily weighted toward false positives. Even the current gold standard of biopsy results in varying diagnoses among pathologists. We have developed a multiphoton technique (based on pump-probe spectroscopy) that directly determines the microscopic distribution of eumelanin and pheomelanin in pigmented lesions of human skin. Our initial results showed a marked difference in the chemical variety of melanin between nonmalignant nevi and melanoma, as well as a number of substantial architectural differences. We examined slices from 42 pigmented lesions and found that melanomas had an increased eumelanin content compared to nonmalignant nevi. When used as a diagnostic criterion, the ratio of eumelanin to pheomelanin captured all investigated melanomas but excluded three-quarters of dysplastic nevi and all benign dermal nevi. Additional evaluation of architectural and cytological features revealed by multiphoton imaging, including the maturation of melanocytes, presence of pigmented melanocytes in the dermis, number and location of melanocytic nests, and confluency of pigmented cells in the epidermis, further increased specificity, allowing rejection of more than half of the remaining false-positive results. We then adapted this multiphoton imaging technique to hematoxylin and eosin (H&E)–stained slides. By adding melanin chemical contrast to H&E-stained slides, pathologists will gain complementary information to increase the ease and accuracy of melanoma diagnosis.