Melanoma & Mohs Surgery: Indications and Slow Mohs Technique

Melanoma arises from the malignant transformation of melanocytes and is the most lethal form of skin cancer. An estimated 100,640 new cases of invasive melanoma and approximately 97,610 cases of melanoma in situ were diagnosed in the United States in 2024. Melanoma causes approximately 8,290 deaths annually in the US, accounting for the vast majority of skin cancer mortality. The incidence has been rising steadily for decades, with a particularly sharp increase in melanoma in situ diagnoses. Ultraviolet radiation exposure. Both chronic cumulative and intermittent intense (blistering sunburns). Is the primary environmental risk factor. Fair skin, multiple nevi (>50 common nevi or >5 atypical nevi), family history of melanoma, personal history of prior melanoma, and genetic mutations (CDKN2A, CDK4, MC1R variants) all confer elevated risk. Unlike nonmelanoma skin cancers, melanoma has significant metastatic potential even at relatively thin depths, making early detection and adequate surgical margins critical to survival outcomes. The 5-year survival rate ranges from >99% for localized, thin melanoma to <30% for distant metastatic disease.

Melanoma is classified into several histopathologic subtypes, each with distinct clinical behavior, anatomic predilection, and molecular features. Superficial spreading melanoma (SSM) is the most common subtype, accounting for approximately 70% of all melanomas. It is characterized by an initial radial growth phase with intraepidermal spread of atypical melanocytes before transitioning to a vertical growth phase with dermal invasion. Nodular melanoma (NM) accounts for 15-20% of melanomas and is characterized by a dominant vertical growth phase from the outset, lacking a significant radial growth phase. This leads to rapid thickness increase and a disproportionately high mortality contribution. Lentigo maligna (LM) / lentigo maligna melanoma (LMM) occurs on chronically sun-damaged skin, predominantly in elderly patients, and is characterized by an often prolonged radial growth phase with atypical melanocytes along the dermoepidermal junction. LM is the subtype most amenable to Mohs surgery due to its superficial nature and frequent location on the face. Acral lentiginous melanoma (ALM) occurs on palms, soles, and nail beds, accounts for a small percentage of melanomas in fair-skinned populations but is the most common subtype in patients with darker skin.

The NCCN v1.2026 guidelines provide evidence-based recommendations for surgical excision margins based on Breslow thickness. These margin recommendations were established through multiple randomized controlled trials (including the Intergroup Melanoma Surgical Trial, the Swedish Melanoma Study, and the UK MelaST trial) comparing narrow vs. wide margins. The margins represent a balance between oncologic adequacy (minimizing local recurrence) and tissue conservation. For melanoma in situ, margins of 0.5-1.0cm are recommended, though expert consensus rather than RCT data supports this range. For invasive melanoma <=1.0mm (T1), 1cm margins are recommended and have been shown to be equivalent to wider margins in randomized trials. For melanoma 1.01-2.0mm (T2), 1-2cm margins are recommended, with RCTs demonstrating equivalent melanoma-specific survival for 1cm vs. 2cm margins. For melanoma >2.0mm (T3/T4), 2cm margins are recommended; margins greater than 2cm have not demonstrated additional benefit. These margins are measured clinically from the visible tumor edge or biopsy scar to the planned incision line. These margin guidelines assume histologic clearance on permanent sections. If margins are positive, re-excision or Mohs should be considered. For melanoma on the face, particularly melanoma in situ/lentigo maligna, strict adherence to margin guidelines may result in unacceptable cosmetic or functional outcomes, and margin-controlled techniques (slow Mohs, staged excision) provide a tissue-sparing alternative.

The use of Mohs micrographic surgery for melanoma has been a subject of significant debate and evolving practice. Traditional (frozen section) Mohs is limited for melanoma because individual melanocytes at the tumor periphery can be extremely difficult to identify on standard H&E-stained frozen sections. The distinction between malignant melanocytes, normal background melanocytes, and sun-damage-associated melanocytic hyperplasia can be challenging even on permanent sections. For this reason, the primary indication for margin-controlled excision of melanoma has shifted to the "slow Mohs" or staged excision technique using immunohistochemistry on permanent or rush permanent sections. The strongest indication for slow Mohs is melanoma in situ (MIS) and lentigo maligna (LM) on the head and neck, particularly on the nose, eyelids, ears, and lips, where standard WLE margins would result in significant tissue loss. NCCN v1.2026 recognizes margin-controlled staged excision (including slow Mohs) as an acceptable approach for melanoma in situ, particularly when tissue conservation is important. The technique provides complete margin assessment with immunohistochemical verification, achieving high cure rates (>99% for MIS) while minimizing the size of the surgical defect.

The slow Mohs (staged excision with rush permanent sections and immunohistochemistry) technique is specifically designed to address the limitations of traditional frozen-section Mohs for melanoma. The procedure is performed over multiple days, with the patient returning for each stage after the histologic results from the previous stage are available. On Day 1, the clinically visible melanoma (or biopsy scar) is debulked, and a thin peripheral layer (typically 2-3mm margin) is excised with beveled edges, divided into mapped sections, and submitted for rush permanent processing with immunohistochemistry. The tissue undergoes formalin fixation, paraffin embedding, and sectioning (typically overnight or within 24-48 hours for rush processing). Immunohistochemical staining with MART-1/MelanA, SOX10, and/or HMB-45 is performed to identify melanocytes at the surgical margin. The Mohs surgeon examines the sections under the microscope, evaluating the entire peripheral margin for atypical melanocytes. If residual melanoma is identified, the patient returns for a targeted re-excision of only the positive area, and the new tissue is processed identically. This cycle repeats until all margins are clear. Some centers have developed same-day rapid immunostaining protocols that allow completion of slow Mohs in a single day, using optimized fixation and staining techniques that produce results within 4-6 hours.

Sentinel lymph node biopsy (SLNB) is a well-established staging procedure for melanoma that identifies occult nodal metastases before they become clinically apparent. NCCN v1.2026 recommends discussing and offering SLNB for melanomas with Breslow thickness >0.8mm, as well as for thinner melanomas (T1a, 0.8mm or less) when adverse features are present, including ulceration, high mitotic rate (>=2 mitoses/mm2), lymphovascular invasion, or young patient age. SLNB provides critical staging information that determines eligibility for adjuvant systemic therapy and guides follow-up intensity. The overall positive SLNB rate for melanoma is approximately 15-20% for intermediate-thickness tumors (1-4mm). For thin melanomas (<=1mm) the positive rate is approximately 5-8%, and for thick melanomas (>4mm) it is approximately 30-40%. SLNB should be performed at the time of wide local excision to ensure accurate lymphatic mapping. Prior wide excision disrupts the lymphatic drainage pathways and reduces SLNB accuracy. When SLNB is positive, completion lymph node dissection (CLND) is no longer routinely recommended based on the DeCOG-SLT and MSLT-II trials showing no survival benefit for immediate CLND. Instead, nodal basin surveillance with ultrasound and consideration of adjuvant systemic therapy is the standard approach. For head and neck melanomas, SLNB accuracy may be reduced due to the complex lymphatic drainage patterns in this region, with drainage to multiple nodal basins (parotid, cervical levels I-V, occipital, postauricular). Lymphoscintigraphy with SPECT/CT is recommended to identify all draining basins preoperatively.

Adjuvant systemic therapy for melanoma has undergone a revolution with the introduction of immune checkpoint inhibitors and BRAF/MEK-targeted therapy. NCCN v1.2026 recommends adjuvant therapy for patients with resected Stage IIB-IV melanoma, with specific agent selection depending on the tumor stage and molecular profile. Anti-PD-1 immune checkpoint inhibitors are the backbone of adjuvant melanoma therapy. Nivolumab (Opdivo) was the first anti-PD-1 agent approved in the adjuvant setting based on the CheckMate-238 trial, demonstrating superior recurrence-free survival compared to ipilimumab. Pembrolizumab (Keytruda) demonstrated significant improvement in recurrence-free survival for Stage III melanoma in the KEYNOTE-054 trial and for Stage IIB/IIC in KEYNOTE-716. For patients with BRAF V600E/K-mutant melanoma, the combination of dabrafenib (Tafinlar) and trametinib (Mekinist). A BRAF inhibitor plus MEK inhibitor. Provides an alternative adjuvant option based on the COMBI-AD trial. The choice between immunotherapy and targeted therapy in BRAF-mutant melanoma involves weighing efficacy, toxicity profile, treatment duration (typically 1 year for both), and patient preference. Neoadjuvant immunotherapy (pre-surgical treatment) is an emerging approach for clinically node-positive melanoma, with promising pathologic complete response rates in recent trials.

Melanoma prognosis is determined by the AJCC 8th edition staging system, which integrates Breslow thickness, ulceration, mitotic rate, nodal status, and distant metastasis. Survival rates have improved significantly in the modern era due to earlier detection, SLNB staging, and the availability of effective adjuvant systemic therapy. Stage IA melanoma (T1a N0, <=0.8mm without ulceration) has a 5-year melanoma-specific survival exceeding 99%, while Stage IV melanoma (distant metastasis) has a 5-year survival of approximately 20-30%. Though this has improved dramatically with the introduction of immunotherapy and targeted therapy. The Breslow thickness remains the single most powerful prognostic factor for localized melanoma. Ulceration is an independent adverse prognostic factor at every T-stage. Sentinel lymph node status is the most important prognostic factor for patients without clinically evident nodal disease.