p16 Range of expression in dermal predominant benign epithelioid and spindled nevi and melanoma
Gabriel Oaxaca, Steven D. Billings, Jennifer S. Ko
Abstract
BACKGROUND: Screening borderline Spitz tumors with p16 immunohistochemistry (IHC) has known utility. The applicability to other melanocytic neoplasms is not well defined. METHODS: 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. RESULTS: 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. CONCLUSION: 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.