Explainer · August 9, 2026 · 5 min · By Hector Lindelof
Mohs for Melanoma: What Immunostains Changed and What They Did Not
Mohs surgery was built for basal and squamous cell carcinomas. A staining technique developed over the past two decades has pushed it into melanoma territory. Here is what the mechanism actually supports, and where the debate still sits.
For most of its history, Mohs micrographic surgery had a hard boundary: it worked for basal cell carcinoma and squamous cell carcinoma, and it was considered unreliable for melanoma. That boundary has shifted, and the reason is not a new scalpel or a new microscope. It is a chemistry change in how tissue is stained during the procedure. Understanding that change explains both why some surgeons now use Mohs for certain melanomas and why others remain cautious.
The original problem was visibility, not technique. Mohs works by removing a thin layer of tissue, freezing it, slicing it horizontally, staining it, and reading the entire margin under a microscope while the patient waits. The standard stain, hematoxylin and eosin, colors cell structures in blues and pinks. Basal cell and squamous cell carcinomas form clusters and strands that stand out clearly against normal skin on frozen sections. Melanoma cells behave differently. Individual atypical melanocytes can scatter along the basal layer of the epidermis, one cell at a time, in a pattern called lentiginous spread. On a frozen section stained with hematoxylin and eosin, a single abnormal melanocyte looks very much like a normal one, or like a keratinocyte with freezing artifact. Freezing also distorts cells and can create clear spaces around keratinocytes that mimic melanocytes. The result was an unacceptable risk of reading a margin as clear when it was not.
Immunostains changed the visibility problem directly. Immunohistochemistry uses antibodies that bind to proteins found specifically in melanocytes, then attaches a colored marker to those antibodies. The most widely used target in Mohs labs is MART-1, also called Melan-A, a protein on the melanocyte cell surface. Other options include SOX10, a nuclear transcription factor, and MITF. When a frozen section is treated with a MART-1 stain, every melanocyte on the slide turns a distinct reddish brown, regardless of freezing artifact. The surgeon is no longer trying to distinguish subtle nuclear features. The question becomes simpler: how many stained cells are present, where are they, and are they arranged in a way that indicates tumor rather than normal background melanocytes. Rapid protocols now allow this staining to be completed in under an hour, which keeps the same-day workflow that defines Mohs intact.
Where the evidence is strongest. The best-supported use of Mohs with immunostaining is melanoma in situ, particularly the lentigo maligna subtype that occurs on chronically sun-damaged skin of the head and neck. These tumors are notorious for subclinical extension, meaning the abnormal melanocytes spread well beyond what is visible to the eye. Studies of standard excision for lentigo maligna have shown that the traditional 5 millimeter margin fails to clear the tumor in a meaningful fraction of cases, which is why guidelines for wide excision of melanoma in situ now describe margins of 5 to 10 millimeters. Because Mohs examines essentially 100 percent of the peripheral margin rather than the small representative slices used in standard pathology, it is well suited to a tumor whose main threat is unpredictable lateral creep. Published series report low local recurrence rates for melanoma in situ treated with immunostain-assisted Mohs, and current appropriate use criteria in the United States support Mohs for melanoma in situ in anatomically constrained sites.
Where caution remains. Invasive melanoma is a different discussion. The concern with deeper melanoma is not only the local margin but the risk of metastasis, and the depth of invasion drives decisions about sentinel lymph node biopsy. Some surgeons argue that removing the tumor with Mohs before nodal staging could theoretically alter lymphatic drainage patterns, although data on this are mixed. There is also a practical issue: interpreting immunostained frozen sections of sun-damaged skin requires judgment, because chronically sun-exposed skin normally contains an increased density of melanocytes. A stained slide full of brown cells is not automatically tumor. Distinguishing background melanocyte hyperplasia from true lentiginous melanoma spread is a genuine skill, and thresholds such as melanocyte counts per high-power field and the presence of nesting or pagetoid spread are used, but they are not perfectly standardized across labs.
What patients should take from this. If you have been diagnosed with melanoma in situ on the face, scalp, ears, or another tight anatomic site, Mohs with immunostaining is a legitimate, evidence-supported option, and its main advantage is complete margin evaluation with tissue conservation. If you have invasive melanoma, standard wide excision with formal staging remains the default in most settings, and any Mohs-based approach should come with a clear explanation of how depth, staging, and pathology review will be handled. A related technique called staged excision, sometimes described as slow Mohs, uses permanent rather than frozen sections read over one to two days and offers similar complete margin control with the more familiar staining conditions of a standard pathology lab.
The short version: immunostains did not make melanoma behave like basal cell carcinoma. They made melanoma cells visible on frozen sections, which solved the specific technical failure that kept Mohs out of melanoma care. The biology of the disease, and the staging questions that come with invasion, still set the limits.
Related reading: Can Mohs surgery treat melanoma?.
Further reading: Mohs Micrographic Surgery With Melanocytic Immunostains for T1a/b Invasive Melanoma Yields <1% Local Recurrence and Disease-specific Mortality (Dermatol Surg 2025); Comparison of staged excision and Mohs micrographic surgery with and without MART-1 immunostains for surgical treatment of melanoma of the head, neck, and special sites: A retrospective cohort study (J Am Acad Dermatol 2021); Interrater and Intrarater Reliability of Mohs Surgeons in the Assessment of MART-1 Frozen Section Margins for Melanoma (Dermatol Surg 2026).