p16 Range of expression in dermal predominant benign epithelioid and spindled nevi and melanoma

Screening borderline Spitz tumors with p16 immunohistochemistry (IHC) has known utility. The applicability to other melanocytic neoplasms is not well defined.

Cases (N = 104) of blue, cellular blue, epithelioid blue, congenital pattern, deep penetrating, desmoplastic, desmoplastic Spitz, acral, "epithelioid" nevi, nevoid melanoma, melanoma with a precursor nevus, and non-nevoid melanoma with Breslow thickness > 0.5 mm were stained for p16.

Lesions showed either a single uniform pattern of expression (single/homogeneous pattern: positive, checkerboard, rare, or lost) or multiple regionally distributed patterns (multiple/heterogeneous pattern). Most cases (78%, n = 81) showed single pattern expression. Within single pattern cases, total loss was restricted to melanoma (7/81/9%). Multiple patterns were more common in melanoma (12/23, 52%). Within multiple pattern (22%, n = 23) lesions, those with a total loss component (7/23; 30%) were malignant. Total p16 loss (diffuse or regional) was not seen in a subset of nevoid melanomas (1/8; 12.5%), melanomas arising in nevi (2/6; 33%), and non-nevoid melanomas (6/9; 66%). Total p16 loss (single pattern or part of multiple patterns) captured 61% (14/23) of melanomas and no nevi.

p16 IHC may be useful in dermal-based melanocytic lesions. Total p16 loss is seen only in melanoma. Multiple pattern expression should prompt careful evaluation.