Verrucous papule on thigh
Was this lesion the result of sun exposure, or a recent trauma to the patient’s leg?
A mimicker of melanoma
An angiokeratoma is an uncommon, though important, mimicker of melanoma. (For more on other lesions that can be confused with melanoma, see “Nonmelanocytic melanoma mimickers”.)
Melanoma is the most aggressive and potentially life-threatening neoplasm in the differential diagnosis of an angiokeratoma. Risk factors for melanoma include increasing age, fair skin and hair color, tendency for freckling, number of moles (5 large or >50 small nevi doubles the risk of melanoma), a personal or first-degree family history of melanoma, and a history of intermittent sunburns.9-12
A number of nonmelanocytic lesions can be confused with melanoma. They include the following:
Actinic keratoses (AKs) are a type of keratinocytic neoplasm that typically develops on the sun-exposed skin of the elderly. An AK is typically 3 to 10 mm in size, pink to red in color, and has scaling secondary to local hyperkeratosis. If these lesions are left untreated, they can develop into squamous cell carcinomas (SCCs) at a rate of 0.24% annually.15,16 Thus, AKs are more often a concern for SCC than for melanoma. However, the pigmented variant of an AK can clinically and histologically raise concern for melanoma due to its pigmentation and microscopic evidence of melanin within keratinocytes and macrophages.15 If it is not possible to differentiate an AK from melanoma clinically or histologically, immunohistochemistry is often required to make the final diagnosis. For example, immunohistochemical staining with S-100 can be used to identify epidermal melanocytes and distinguish them from atypical keratinocytes.17
Basal cell carcinoma (BCC) is the most common skin cancer.18 While most BCCs are amelanocytic, 7% of BCCs are pigmented and present as irregularly pigmented nodules with irregular telangiectatic vessels on their surface. The center of a BCC may be depressed or ulcerated and may easily crust or bleed. Definitive diagnosis may be made histologically. A BCC typically consists of columns of basaloid cells with atypical nuclei, sparse cytoplasm, and peripheral cellular palisading.19 BCCs are easily differentiated from melanoma using immunohistochemistry, as they are negative for traditional melanocytic markers.17
Seborrheic keratoses (SKs) are among the most common skin lesions and represent a benign proliferation of immature keratinocytes. The appearance of an SK can vary from a smooth peppered appearance to a rough surface that may be irregularly pigmented, dry, and fissured. Given their range of presentation, it is common for SKs to be biopsied to evaluate for melanoma and occasionally BCC.20
Dermatofibromas (DFs) are common benign skin lesions that typically appear as pink-to brown-colored firm nodules that represent a localized response to skin injury and inflammation. DFs are typically 3 to 10 mm in diameter and are most commonly located on the anterior surface of the thigh. Histologic analysis of a DF reveals an acanthotic epidermis with a proliferation of spindle cells in the mid and lower dermis, with capillaries dispersed throughout. A common finding in DFs is the trapping of collagen within the spindle cell at the periphery of the lesion.21
