This educational video by dermatopathologist Dr. Jared Gardner provides a comprehensive guide to using immunohistochemical (IHC) stains for the diagnosis of melanocytic lesions. Dr. Gardner emphasizes that while traditional H&E staining remains the primary diagnostic tool, IHC serves as a crucial adjunct to clarify histological patterns in challenging cases. The video begins with an overview of preferred stains for specific scenarios, consistently favoring the crisp nuclear stain Sox10 for assessing epidermal components over cytoplasmic stains like Mart-1 (which can appear misleadingly "busy") and less specific markers like MITF and S100. Through a series of practical case examples ranging from benign nevi to melanoma in situ and nodular melanoma, the presentation demonstrates how to interpret these stains effectively. Key teaching points include identifying diagnostic pitfalls like tangential sectioning artifacts, differentiating melanoma from solar-induced melanocytic hyperplasia on sun-damaged skin, and understanding the variable expression of markers like Mart-1 in metastatic or nodular melanoma.
A Dermatopathologist's Guide to Immunohistochemical Stains in Melanocytic Lesions
0:00:02 H&E Staining is King: The speaker stresses that immunohistochemical stains are an adjunct tool to help assess histological patterns. The primary diagnosis for the vast majority of melanocytic lesions should be based on the H&E stain.
0:02:42 Recommended Stains for Different Scenarios: Dr. Gardner outlines his preferred stains for various diagnostic challenges:
Pagetoid Spread/Confluent Growth: Sox10 is the best choice due to its crisp nuclear staining. Mart-1 can be misleadingly busy, and S100 is "messy" as it also highlights Langerhans cells.
Metastatic & Spindle Cell Melanoma: S100 and Sox10 are the most sensitive markers. Mart-1 and HMB45 are often lost in these melanomas.
Desmoplastic Melanoma: Sox10 and S100 are the go-to stains, as these tumors are almost always negative for other melanocytic markers.
Sentinel Lymph Nodes: Sox10 and Mart-1 are preferred for their clean and specific staining. S100 and MITF are strongly discouraged due to nonspecific staining of other cells in the lymph node.
0:09:41 Comparing Key Stain Characteristics: The video provides a foundational overview of the most common stains using an intradermal nevus as an example.
S100: A very sensitive but nonspecific cytoplasmic and nuclear stain. It stains Langerhans cells, making it difficult to interpret junctional components.
Mart-1 (Melan-A): A crisp cytoplasmic stain that highlights dendritic processes. This can make the lesion look artificially "busy" and confluent.
HMB45: A cytoplasmic stain that is often lost as nevi "mature" (go deeper). It is generally less sensitive than Mart-1.
Sox10: The speaker's preferred marker. It is a clean, crisp nuclear stain that is excellent for visualizing the precise number and pattern of melanocytes.
0:20:01 Diagnostic Pitfall: Misleading Cytoplasmic Stains: A case of lentigo simplex demonstrates how Mart-1's cytoplasmic staining of dendritic processes can create a false impression of confluent growth, especially with tangential sectioning. In contrast, the nuclear Sox10 stain clearly shows the benign, spaced-out pattern of melanocytes.
0:38:34 The "Ben Wood Trick" for Assessing Lesions: A helpful rule of thumb is introduced: if you draw a line across the top of the dermal papillae, and most melanocytes are below that line (in the rete ridges), the lesion is more likely to be benign (e.g., a dysplastic nevus).
0:50:19 Using IHC to Reveal "Occult" Melanoma: Several examples show how lesions that appear to be benign junctional nevi on H&E, particularly on the sun-damaged skin of older individuals, can be revealed as melanoma in situ with a Sox10 stain, which uncovers extensive confluent growth not visible otherwise.
0:58:15 Diagnostic Pitfall: Solar-Induced Melanocytic Hyperplasia: In sun-damaged skin, there is often a background increase in the number and size of melanocytes. This can be difficult to distinguish from melanoma in situ at the margins of an excision. Key features of this benign change are a relatively uniform, spaced-out distribution of melanocytes across the entire specimen, in contrast to the more crowded and atypical cells of a true melanoma.
1:11:08 Staining Patterns in Dermal/Nodular Melanoma: A final case illustrates IHC patterns in a purely dermal nodule of melanoma.
Key Takeaway: Large nodules of melanoma (primary or metastatic) often lose expression of Mart-1 and HMB45. Therefore, if a suspected melanoma is negative for Mart-1, it is critical to follow up with the more sensitive S100 and Sox10 stains, which are usually retained.