Minimal deviation melanoma: a histologic variant of cutaneous malignant melanoma in its vertical growth phase.

Minimal deviation melanomas are uncommon nevo-melanocytic tumors of indeterminate risk that appear as pigmented or nonpigmented skin nodules and are clinically diagnosed as Spitz nevi, hemangiomas, or malignant melanomas. They are characterized histologically by expansile growth in the papillary dermis with reticular dermal infiltration (minimal deviation type) or without reticular dermal invasion (borderline type). The tumors exhibit lesser cytologic atypia in their vertical growth phase (histologic variance) than observed in common forms of melanoma. A retrospective study of outcome in 21 patients with minimal deviation melanoma (mean tumor thickness by Breslow's measurement=3.6 mm) disclosed recurrent disease in only 3 patients after a mean observation period of 57 months, supporting the impression that these tumors are not as biologically aggressive as common malignant melanomas. The histologic subtypes of minimal deviation melanoma are reviewed along with a discussion of the concept of histologic variance.