Immunohistochemistry in Mohs Surgery: IHC Stains & Markers

Immunohistochemistry adds antibody-based staining to Mohs frozen sections when standard H&E cannot reliably identify tumor cells at the surgical margin. The primary indications for IHC in Mohs are melanocytic lesions (lentigo maligna, melanoma in situ, invasive melanoma), perineural invasion detection, and rare adnexal or dermal tumors with bland histologic features. H&E alone is adequate for identifying most basal cell carcinomas and squamous cell carcinomas on frozen sections. However, melanocytes at the periphery of lentigo maligna are single, scattered cells in a background of sun-damaged skin with melanocytic hyperplasia — making them nearly impossible to distinguish from benign melanocytes on H&E frozen sections. IHC solves this by highlighting melanocytes selectively, allowing the surgeon to count and assess individual cells at the margin. The development of rapid IHC protocols that complete staining in 20-45 minutes has transformed Mohs for melanoma from a multi-day staged procedure into a same-day operation at high-volume centers. Modern automated staining platforms further reduce hands-on time and improve reproducibility.

MART-1 (Melanoma Antigen Recognized by T-cells 1), also marketed as Melan-A, is the most widely used melanocyte marker in Mohs surgery worldwide. It is a cytoplasmic stain that labels melanocytes with brown chromogen (DAB), producing a dendritic pattern that outlines individual melanocytes along the dermoepidermal junction. MART-1 has 95-100% sensitivity for melanoma on frozen sections and is reactive in virtually all subtypes of melanoma in situ and lentigo maligna. The antibody labels both malignant and benign melanocytes, which is both its strength (high sensitivity) and its weakness (limited specificity in sun-damaged skin). On MART-1-stained sections, the Mohs surgeon counts melanocytes per linear millimeter of epidermis to determine whether the margin contains a pathologic density of melanocytes. Normal non-sun-damaged skin contains approximately 5-10 melanocytes per mm of epidermis. Chronically sun-damaged skin (face, scalp, dorsal hands) often shows baseline melanocytic hyperplasia of 15-25 melanocytes per mm even without melanoma. Lentigo maligna margins typically show confluent melanocytes exceeding 25-40 per mm, often with pagetoid scatter, nesting, and cytologic atypia.

SOX10 (SRY-related HMG-box 10) is a nuclear transcription factor expressed in melanocytes and Schwann cells. Unlike MART-1, which produces a cytoplasmic stain with dendritic processes, SOX10 produces a crisp nuclear signal with minimal background noise. This cleaner staining pattern has made SOX10 increasingly preferred for Mohs surgery of lentigo maligna, particularly in high-volume melanoma Mohs centers. SOX10 sensitivity for melanoma ranges from 95-100%, comparable to MART-1. Its key advantages over MART-1 include: 1. Nuclear staining produces a discrete dot rather than a diffuse cytoplasmic blush, making individual melanocytes easier to count and reducing interobserver variability. 2. No dendritic process artifact — MART-1 labels the entire dendritic tree of melanocytes, which can create a falsely busy appearance and make adjacent cells difficult to distinguish as separate entities. 3. Simultaneous detection of perineural invasion — SOX10 labels Schwann cells, so perineural tumor involvement is highlighted on the same slide used for melanocyte assessment. 4. Reliable in desmoplastic melanoma — SOX10 maintains 95-100% sensitivity for desmoplastic melanoma, where MART-1 fails in 50-70% of cases. The primary limitation of SOX10 is that it also labels Schwann cells and other neural crest-derived cells in the dermis. In heavily innervated areas (nasal tip, lips, periorbital skin), background dermal SOX10 positivity from Schwann cells can complicate interpretation.

PRAME (Preferentially Expressed Antigen in Melanoma) is a cancer-testis antigen that is expressed in melanoma but absent in most benign melanocytic nevi. This specificity for malignancy distinguishes PRAME from both MART-1 and SOX10, which label all melanocytes regardless of their biologic behavior. PRAME shows 87-94% sensitivity for melanoma and approximately 96% specificity (meaning only 4% of benign nevi are PRAME-positive). In the context of Mohs surgery, PRAME has the potential to address the most significant limitation of MART-1: false-positive margins caused by benign melanocytic hyperplasia in sun-damaged skin. When PRAME is positive at a Mohs margin, it provides strong evidence that the melanocytes present are malignant rather than reactive. When PRAME is negative, the margin melanocytes are more likely benign hyperplasia — though a negative result does not completely exclude melanoma, given the 6-13% false-negative rate. PRAME is not yet standard in Mohs frozen section laboratories for several reasons: the antibody performs less reliably on frozen tissue compared to formalin-fixed paraffin-embedded (FFPE) sections, rapid frozen IHC protocols for PRAME are still being optimized, and clinical outcome data validating PRAME-guided margin decisions in Mohs are limited. However, PRAME is increasingly used as an adjunct in slow Mohs protocols that use permanent sections, and its adoption in rapid frozen Mohs is expected to grow.

The following table summarizes the key IHC markers used in Mohs surgery, their targets, diagnostic performance, and optimal applications.

Beyond melanocyte markers, several other IHC stains serve specific roles in Mohs micrographic surgery for non-melanocytic tumors and special situations.

Slow Mohs (also called staged excision with permanent section margin control) is a modified Mohs technique used primarily for melanoma in situ and lentigo maligna. Instead of processing tissue as frozen sections with rapid IHC during a single-day procedure, slow Mohs uses formalin-fixed permanent sections with IHC (MART-1 or SOX10) processed over 1-2 days between stages. The slow Mohs protocol typically follows this sequence: 1. Stage 1: Excise the lesion with 5-10 mm clinical margins. Close the defect temporarily (primary closure, purse-string, or wound dressing) or leave it open. 2. Process the peripheral margin with en face permanent sections stained with MART-1 or SOX10. 3. Wait 1-5 days for permanent section results. 4. If margins are positive, return for Stage 2: re-excise only the positive margin segment with an additional 2-5 mm. 5. Repeat until margins are clear. 6. Final reconstruction after confirmed clear margins. Slow Mohs achieves 97-99% clearance rates for lentigo maligna and melanoma in situ. The primary advantage over rapid frozen IHC is superior tissue quality on permanent sections — FFPE tissue produces cleaner IHC staining with fewer artifacts. The primary disadvantage is the multi-day timeline and the need for temporary wound management between stages.

Performing IHC on frozen sections presents technical challenges that differ substantially from standard formalin-fixed histopathology IHC. Understanding these differences is essential for establishing and maintaining a reliable IHC program in the Mohs laboratory.

Interpreting IHC-stained frozen sections in Mohs surgery requires a systematic approach that accounts for the technical limitations of frozen tissue and the clinical context of margin assessment. The goal is not histopathologic diagnosis — the tumor has already been biopsied — but rather binary margin assessment: are malignant cells present at the surgical edge?

Several emerging technologies are poised to change how IHC is performed and interpreted in Mohs surgery over the next 5-10 years.