Diagnostic Errors in Patients With Rashes, Moles, and Other Skin Findings

Graeme M. Lipper, MD


September 06, 2017

A 'Changing Mole'

An 82-year-old white woman presents with a "suddenly changing/darkening mole" on her right scapular back, which she describes as "black, irregular, and slightly raised." Her husband noticed the lesion 3 days ago during a trip to the beach. The day before noticing the mole, the patient had applied self-tanning lotion and denies any recent sunburns. Her medical history is significant for a basal cell carcinoma on her nose 5 years earlier (treated with Mohs micrographic surgery) and facial actinic keratoses. Her family history is negative for melanoma or atypical nevi. Examination reveals a dark-brown-to-black, irregularly pigmented, 1.2-cm plaque on her right scapular back, with scattered smaller tan-to-brown macules and papules scattered on her torso and extremities (Figure 1).

Figure 1. Patient's scapular lesion. Image courtesy of Graeme Lipper, MD.

She is referred to a plastic surgeon for wide local excision of a suspected malignant melanoma.

Why is this a medical diagnostic error?

Evaluating a Pigmented Lesion

Changing pigmented lesions are a common source of alarm for patients and healthcare providers, with malignant melanoma as a primary concern. In this case, the pigmented lesion in question changed color, prompting an urgent evaluation. However, the lesion was a seborrheic keratosis—a benign mimic of melanoma that requires no treatment.

Seborrheic keratosis is the most common benign epithelial neoplasm, with hereditary predisposition and lesions ranging from small subtly raised papules to thick plaques with a "stuck-on" appearance. Scale and color vary; lesions can range from smooth to verrucous and pink to black. In this case, the sudden darkening of the lesion was due to application of a dihydroxyacetone (DHA)-based self-tanner. Temporary darkening was induced by a chemical reaction between DHA and amino acids in keratin. Hair dyes can cause similar sudden darkening of seborrheic keratoses on the scalp, neck, and hairline.

Elliptical excision in this case would cause unnecessary scarring with the potential for postsurgical complications including infection, wound dehiscence and unnecessary patient anxiety. This can be avoided with simple dermoscopic (epiluminescence microscopy) evaluation of the lesion, which would reveal the diagnostic features of seborrheic keratosis: homogeneous pigmentation with absence of a pigment network, comedo-like openings, milia, and/or a verrucous or cerebriform surface.[1] In contrast, dermoscopic exam of an malignant melanoma (Figure 2) would show an irregular pigment network, blue-white veil, irregularly dispersed globules and dots, pseudopods or streaks, and/or blue-black sign. Equivocal lesions or those with dermoscopic features of both seborrheic keratosis and pigmented melanocytic nevi should always be biopsied, either with a deep shave or excisional technique.

Figure 2. Malignant melanoma lesion. Image courtesy of Graeme Lipper, MD.

Other benign heavily pigmented lesions mimicking malignant melanoma include blue nevi (homogeneous blue-gray pigmentation) and ink-spot lentigines (lacelike reticular pigment network).


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