Diagnostic Considerations
Melanoma is one of the great clinical imitators and can manifest with great clinical diversity. Failure to have a high index of suspicion and failure to evaluate new or changing lesions that are not unequivocally benign via biopsy, with subsequent interpretation by a competent, certified dermatopathologist, can lead to liability.
Histopathological overdiagnosis refers to the interpretation of a benign lesion as melanoma, which is a tendency of some pathologists. Overdiagnosis can prompt detailed and unwarranted clinical investigation of the patient, can provoke patient anxiety with respect to longevity, and can trigger unnecessary surgery, including reexcision and lymph node sampling. Unnecessary surgery and complications such as chronic lymphedema may result.
Histopathologic underdiagnosis refers to the interpretation of melanoma as a benign lesion. Underdiagnosis can yield considerable delays that may permit metastatic spread before appropriate extirpation is possible.
Clinicians and patients can avoid clinical and histopathological misdiagnosis by adhering closely to standard practices in the evaluation of melanocytic neoplasms and by seeking appropriate consultations with experienced colleagues.
The prototypical melanoma is readily diagnosable by the ABCDE approach, based on its asymmetry, irregular border, irregular color, large diameter, and evolution. However, these clinical parameters are largely useless in three instances, as follows:
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For amelanotic melanomas, in which pigmentation is largely or entirely absent
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For desmoplastic melanomas, which sometimes manifest without an associated in situ component and may also lack clinical pigmentation
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For ulcerated and inflamed melanomas, especially nodular lesions, which may clinically simulate common lesions (eg, basal cell carcinoma, pyogenic granuloma) because of masking by the presence of ulceration and/or inflammation
Differential Diagnoses
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Cockarde nevus
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Nevus spilus
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A Clark (dysplastic) nevus with modest variation in pigmentation and irregular borders. Biopsy of the lesion proved no evidence of melanoma.
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A compound Clark (dysplastic) nevus with fried egg–like clinical morphology, with a central dark papule flanked by an eccentric more lightly pigmented macular zone.
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A conventional (papular) melanocytic nevus occurring within acral skin. Note slight border irregularity, a feature common in association with acral nevi.
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A heavily pigmented junctional Spitz nevus, also known as pigmented spindle cell nevus. Note that many Spitz nevi are nonpigmented and may have an angiomalike clinical appearance.
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A melanocytic nevus occurring within conjunctival epithelium.
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A conventional compound melanocytic nevus. Note the presence of melanocytes with small nuclei in nests along the dermoepidermal junction and the presence of similar melanocytes in nests and syncytia in the subjacent dermis.
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This Spitz nevus shows large melanocytes with spindled and epithelioid cytomorphology arrayed along the junctional zone of an acanthotic and hyperkeratotic epithelium.
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At higher magnification, this Spitz nevus also demonstrates large, dull-pink globules along the junctional zone. These structures are known as Kamino bodies. Kamino bodies are most commonly observed in association with Spitz nevi but are occasionally observed in melanocytic nevi of other types. Well-formed Kamino bodies are almost never (if ever) found in association with melanoma.
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This blue nevus is composed of small dendritic melanocytes. This type of cytomorphology can be seen in so-called common blue nevi and in topographically restricted lesions such as nevus of Ito or nevus of Ota.
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This large congenital nevus developed papular areas of pigmentation within it. Microscopic examination proved that the "new" areas represented small nodular collections of benign melanocytes, with no evidence of evolving melanoma.
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Histopathologically, a congenital nevus differs from an acquired melanocytic nevus in that melanocytes are often distributed deeply within the reticular dermis, within the adventitial dermis around adnexal elements, and sometimes within the subcutis. This congenital nevus shows a folliculocentric array of melanocytes.
