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Dr. Phillip McKee's Blog

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Nevoid melanoma

Dr. Phillip McKee


Nevoid melanoma is one of the most important and difficult variants of melanoma. When lecturing, I define it as “a melanoma that you diagnosed as a nevus and wish you hadn’t”! Very commonly, it is a diagnosis of retrospect when a previously diagnosed nevus recurs and the correct diagnosis comes out. I had a mantra when I started my signout reminding myself of conditions I always should bear in mind when looking at the cases- don’t forget nevoid and desmoplastic melanoma (never report a scar without knowing what the previous diagnosis was) and I also added cellular neurothekeoma and myofibroma as they were two tumors that I tended to forget about!
The problem with nevoid melanoma is that it can be so subtle and therefore very easy to miss. It exemplifies the need to look at all nevi carefully and in particular, if anything worries you about a lesion especially late in the day, look at it with a fresh mind in the morning assuming that you have not been out clubbing the night before!

In nevoid melanoma, maturation is always impaired albeit in a subtle fashion. An important point is that it may be focal i.e. some parts of the lesion show full maturation but in other areas it is impaired. Remember that maturation implies a change from type-A cells superficially to type-B or type-C cells at the bottom of the lesion. If the cells in the deep part of the lesion are still type-A cells, then maturation is impaired. At low power examination expansion of the papillary dermis with stretching and thinning of the rete ridges by a diffuse or nodular melanocytic population is a helpful clue.

Mild pleomorphism, slightly larger nucleoli than you would expect and the presence of mitotic figures are obviously additional high magnification clues. It should therefore be obvious that one should never look at “melanocytic nevi” at scanning magnification only. High power examination is critical to the diagnosis of nevoid melanoma. This brings me to the point of what to do when one finds a single mitotic figure in what appears to be an obviously banal melanocytic nevus. The late Wallace Clark taught that 2 mitoses were acceptable in a banal melanocytic nevus and I would be inclined to agree with him provided that they were present in the superficial part of the nevus. Their presence however should initiate further investigation of the lesion. Deeper levels should invariably be examined to look for additional mitoses, deep mitoses or abnormal mitoses. I also used limited immunohistochemistry. MIB1 and Cyclin-D1 can be particularly useful. In a banal nevus only a few cells scattered throughout the most superficial aspect of the lesion should be seen. In nevoid melanoma, there is often diffuse staining throughout the whole extent of the lesion.

All of us have missed nevoid melanoma (I know that I missed one many years ago and perhaps I have missed others but they haven’t come back to haunt me!). Treat all melanocytic nevi with a healthy degree of concern and the likelihood of missing a melanoma will be greatly reduced


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