By Gary M. White, MD
Several Clark nevi. Note the clinical similarities to melanoma.
The Clark nevus, also called dysplastic nevus or B-K mole, is a melanocytic, pigmented lesion with clinical features of melanoma. It may for example have a diameter over 6 mm, irregular colors, shape, etc. The core issue with Clark nevi is that it mimics melanoma both clinically and histologically more frequently than any other nevus. Indeed the diagnostic criteria overlap with melanoma. Unfortunately, histologic differentiation between the Clark nevus and melanoma is difficult. Dermatologists have been struggling with the best clinical approach to Clark nevi for decades. In fact, many dermatologists prefer to ignore the term Clark nevus and merely consider pigmented lesions as clinically typical or atypical with the atypical lesions warranting biopsy (see atypical nevus for more discussion). See also atypical mole syndrome.
The Clark nevus is usually large, with multiple colors and irregular shapes, similar to melanoma. The term is most commonly used in the setting of the atypical mole syndrome. Patients with this syndrome tend to have multiple clinically atypical nevi (e.g. >30) and a familial tendency to develop melanoma.
Two approaches for histologic reading of Clark nevi are generally used.
The most common is to diagnosis the lesion as a nevus with atypia and then grade the atypia as mild, moderate or severe. Nevi with mild atypia (regardless of margin status) and nevi with moderate atypia but negative margins are usually observed. Nevi with moderate atypia and positive margins are usually excised and nevi with severe atypia are almost always reexcised with 3-5 mm margins.
This approach encourages the notion that Clark nevi are somewhere on the biologic continuum between a benign nevus and a melanoma.
The second approach to reading Clark nevi pathologically is to call then Clark nevi and then to have the pathologist recommend reexcision only if s/he is at all uncertain as to whether the lesion could represent a melanoma. This non-grading approach has been advocated as it results in fewer excisions [JAMA 2016;74;68].
This approach encourages the notion that Clark nevi are not direct precursors to melanoma. Any uncertainty reflects our lack of ability to distinguish a Clark nevus from a melanoma under the microscope.
A dermatologist should manage these lesions. Complete removal of any clinically atypical pigmented lesion is recommended. The main difficulty arises when a patient has many, e.g. 10-100. See atypical mole syndrome for more here. A good working relationship with a qualified pathologist is key. Understanding what histologic approach to reading these lesions is used and when the pathologist recommends reexcision is key. When in doubt, remove.
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