Answer of Dermatopathology Case 107

Naevoid Melanoma

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In general, common naevi are diagnosable at low power.
It is important to exclude the possibility of naevoid melanoma.
This is a rare variant that mimics benign naevi and is difficult to recognise.
The correct diagnosis is frequently made only after the patient has developed a metastasis.
At low power, naevoid melanoma may have a nodular architecture, and exhibit other features of a common banal naevus, including circumscription and at least relative symmetry.
Naevoid melanoma lacks the prominent junctional activity and pagetoid spread usually associated with superficial spreading melanoma.
Common intradermal naevus “matures” (there is an overall decrease in nest size and cellular and nuclear size with depth).
On low power examination, naevoid melanoma may appear to mature with depth, yet closer inspection reveals that the cells at the base of the lesions are similar in size to those of the superficial dermal component.
Only at higher power the distinguishing characteristics of naevoid melanoma can be appreciated.
These features include :
1) A monotonous population of small round cells with prominent nucleoli.
2) From a few to numerous mitoses any where in the lesion.
3) Other features that may be present include individual cell necrosis and atypical mitoses.