Enhanced Melan‐A Red Chromogen Labeling in Melanoma Compared to Nevus Cells: A Diagnostic Aid in Dual Multiplex Immunohistochemistry With Ki67

BACKGROUND: Distinguishing melanoma cells from nevus cells remains a diagnostic challenge, particularly in cases where melanoma exhibits nevoid morphology or nevus cells display atypical or epithelioid features. Accurate identification is essential for proper measurement of Breslow depth in melanomas arising in association with nevi and for margin assessment, both of which are critical for patient prognosis. The dual multiplex immunohistochemical stain utilizing Melan-A (red chromogen) and Ki67 (brown chromogen) is a valuable tool for assessing the proliferative index of melanocytic populations, with melanoma cells typically showing a higher Ki67 index than nevus cells. METHODS: We identified 10 cases with invasive melanoma and associated nevus present on the same scanned histologic section for which the signing pathologist had ordered this dual multiplex immunohistochemical study. We annotated areas of melanoma versus nevus, then quantified the staining intensities. RESULTS: Melanoma cells often demonstrate more intense Melan-A red chromogen labeling than adjacent nevus cells (p = 0.008; Wilcoxon signed rank test). CONCLUSION: This differential labeling provides a helpful visual distinction between melanoma and nevus cells within the same histologic section, enhancing diagnostic confidence beyond the difference in Ki67 proliferation index.