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AUSTIN, Texas — It is safe and feasible to treat certain melanoma in situ (MIS) tumors with Mohs micrographic surgery using margins smaller than the standard 5-6 mm typically used, according to a study presented at the American College of Mohs Surgery (ACMS) Annual Meeting 2026.

“The findings show that narrow initial margins can safely be used in an immunohistochemistry-controlled framework, and in our opinion, this can permit tissue conservation, allow for simpler reconstructions, and decreased patient morbidity,” Christian Gronbeck, MD, Mohs Surgery fellow at Zitelli, Brodland & Lim Skin Cancer Center in Pittsburgh, told attendees.

Most published series of Mohs micrographic surgery for MIS have reported cases using initial surgical margins of 5-6 mm, reflecting the early published protocols from the pre-immunohistochemistry (IHC) era, when frozen section assessment without IHC was more difficult, Gronbeck said. With the new confidence provided by IHC in interpreting slides, however, he and his colleagues hypothesized that narrower margins of 4 mm or less should provide the same low local recurrence rates as initial 6 mm margins.

Since incorporating IHC for frozen-margin assessment for MIS in 2004, his center has occasionally used margins of 4 mm or less in select cases. These typically involved early MIS that was adjacent to free anatomical margins and had very small, well-defined lesions, usually less than 1 cm.

In this study, he and his coinvestigators assessed their institution’s outcomes for MIS treated with these narrow margins using Mohs micrographic surgery with IHC-guided margin assessment. They analyzed data from the 530 MIS tumors that were prospectively collected in the center’s melanoma database from January 2004 to September 2025. Their patient entries are maintained annually with medical record review and/or follow-up with patients or managing dermatologists by phone, Gronbeck explained.

The 530 tumors comprised all those with biopsy-proven MIS, including lentigo maligna, treated with Mohs micrographic surgery with a surgical first layer margin of 4 mm or less using IHC. Then they assessed additional clinical, histologic, and operative characteristics for each tumor.

The average patient age was 64 years, and 56.6% were men. The tumors’ average preoperative diameter was 1.24 cm, and cases included 50.4% that were lentigo maligna, 34.5% that were early or evolving based on the biopsy report, and 78.1% with positive margins on the biopsy report.

Most of the tumors were on the face (76%), followed by the extremities (10.6%) and hands and feet (5.6%). A smaller proportion were on the scalp (2.5%), neck (1.3%), and trunk (4%).

The average initial margin for these cases was 3.6 mm, and the average final margin was 4.5 mm, with an average of 1.3 stages required. About 1 in 5 tumors (22.5%) required more than one stage and a final margin greater than 4 mm.

The researchers identified only two characteristics that were associated with the need for a final margin greater than 4 mm. Those patients were typically older (average age, 68.4 vs 63.2 years; P = .0002) and had a larger average preoperative tumor diameter (1.5 cm vs 1.2 cm; P = .0074).

“Interestingly, other factors like lentigo maligna subtype, margin status, and anatomic location were not associated with the need for a greater final margin or more than one stage,” Gronbeck said.

Patients had a median of 3.2 years of follow-up and a 5-year local recurrence-free rate of 100%. The 10-year local recurrence-free rate was 96.1% because of two recurrences (0.4%), both in women patients. One occurred at 5.5 years as a 0.9 mm invasive disease at the proximal nailfold, and the other occurred nearly 9 years later as MIS at the proximal nailfold.

There was one melanoma-related distant metastasis and death (0.2%) that occurred 4.1 years after treatment of a large MIS on the patient’s cheek that extended to their lower eyelid. At the time, it was cleared with a 4 mm margin, “but importantly, we can’t confirm that the metastasis and death here was specifically due to MIS,” Gronbeck said. Based on the published literature, “The likelihood that this was due to a melanoma of unknown primary, we feel, is much more likely than metastasis from this specific MIS.”

Because of that death, the 5-year melanoma-specific survival was 99.4%. Overall survival, meanwhile, was 95.3%.

“Importantly, the findings do reinforce that complete margin assessment is essential since nearly a quarter of the tumors here did eventually require that final margin beyond 4 mm,” Gronbeck said. “As we know from our prior data, if we were to extrapolate this to wide excisions without complete margin control, we’d require final margins of 9-12 mm to clear 97% of tumors.”

“But at the end of the day, we feel that the local recurrence rate here at 0.4% is consistent with our prior literature looking at initial margins of 6 mm, which showed local recurrence rates around 0.3%,” he said. “Ultimately, this argues that histologic clearance, and not modern empirical margins, is the key determinant in tumor control for MIS.”

Jon Meine, MD, Mohs surgeon at the Cleveland Clinic in Cleveland, was not involved in the research and told Medscape Medical News that he found the results of the study simultaneously surprising and encouraging.

“It sounds like it’s safe to use a smaller margin for smaller tumors,” Meine said. If his practice did immunohistochemical stains, he would consider using narrower initial margins as well, he added.

Gronbeck and Meine reported having no disclosures, and no external funding was noted. 

Tara Haelle is a science/health journalist based in Dallas.