Melanocytic lesions of the face: diagnostic pitfalls.

The pathologist often has a difficult task in evaluating melanocytic lesions. For lesions involving the face the consequences of misdiagnosis are compounded for both cosmetic and therapeutic reasons. In this article, the pathological features of common and uncommon benign and malignant melanocytic lesions are reviewed and pitfalls in their diagnosis are highlighted. Benign lesions resembling melanomas include regenerating naevus, "irritated" naevus, combined naevus, "ancient naevus", Spitz naevus, dysplastic naevus, halo naevus, variants of blue naevi, balloon and clear cell naevi, neurotised naevus and desmoplastic naevus. Melanomas that can easily be missed on presentation include desmoplastic, naevoid, regressed, myxoid and metastatic types as well as so-called malignant blue naevi. Pathological clues to benign lesions include good symmetry, V-shaped silhouette, absent epidermal invasion, uniform cellularity, deep maturation, absent or rare dermal mitoses and clustered Kamino bodies. Features more commonly present in melanomas include asymmetry, peripheral epidermal invasion, heavy or "dusty" pigmentation, deep and abnormal dermal mitoses, HMB45 positivity in deep dermal melanocytes, vascular invasion, neurotropism and satellites. Familiarity with the spectrum of melanocytic lesions and knowledge of the important distinguishing features should assist in interpretation. Occasionally, the use of ancillary studies may also provide additional useful information. Application of standard criteria for diagnosing such lesions should enable the correct diagnosis to be established in most cases.