Merkel Cell Carcinoma: Mohs Surgery & NCCN Management

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine tumor arising from mechanoreceptor Merkel cells or their progenitors in the basal layer of the epidermis. The incidence of MCC has increased approximately threefold over the past two decades, currently estimated at 0.7 per 100,000 person-years in the United States, making it roughly 30-40 times less common than melanoma. MCC predominantly affects older individuals (median age at diagnosis 75-80 years) and shows a strong male predominance (approximately 2:1 male-to-female ratio). Ultraviolet radiation is a major etiologic factor, as reflected by the predilection for sun-exposed sites. The head and neck region accounts for approximately 50% of cases, followed by extremities. Merkel cell polyomavirus (MCPyV) is clonally integrated in approximately 80% of MCC tumors and is considered a major oncogenic driver through expression of its large T and small T antigens. UV-associated (virus-negative) MCC tumors carry a very high tumor mutational burden, which has implications for immunotherapy responsiveness. Immunosuppression is a critical risk factor: solid organ transplant recipients carry an approximately 15-fold increased risk, and patients with HIV/AIDS or chronic lymphocytic leukemia are also at substantially elevated risk.

MCC typically presents as a firm, painless, rapidly growing papule or nodule on sun-exposed skin. The lesion is often dome-shaped with a smooth, shiny surface and characteristically displays a red, pink, or violaceous color. The clinical appearance is nonspecific, and MCC is frequently misdiagnosed initially as a cyst, lipoma, or other benign lesion. The AEIOU mnemonic captures the key clinical features: Asymptomatic (lack of tenderness), Expanding rapidly (doubling within 3 months), Immunosuppression, Older than 50 years, and UV-exposed site. Diagnosis requires histopathologic confirmation with immunohistochemistry (IHC), as MCC is a small round blue cell tumor that must be distinguished from other morphologically similar neoplasms. The hallmark IHC pattern is CK20-positive with a characteristic perinuclear dot-like staining pattern, combined with positivity for neuroendocrine markers (synaptophysin, chromogranin A). Critically, MCC is TTF-1 negative, which differentiates it from metastatic small cell lung carcinoma (TTF-1 positive). Additional markers include neuron-specific enolase and CD56. When clinical suspicion for MCC arises, a punch biopsy or incisional biopsy of sufficient depth should be performed, and IHC must be included in the pathology request.

MCC is staged using the AJCC 8th Edition TNM system, which divides cases into stages I through IV. The staging system distinguishes between clinical staging (based on clinical and radiologic assessment) and pathological staging (incorporating sentinel lymph node biopsy results), which is important because pathologic nodal assessment significantly refines prognosis. Primary tumor (T) stage is determined by tumor size: T1 (maximum dimension 2 cm or less), T2 (greater than 2 cm but 5 cm or less), T3 (greater than 5 cm), and T4 (primary tumor invades fascia, muscle, cartilage, or bone). Nodal status is the single most important prognostic factor after primary tumor characteristics. Clinical node-negative patients who undergo SLNB with negative results have significantly better outcomes than those with clinically detected nodal disease. The NCCN v2.2026 guidelines emphasize that SLNB should be performed for all clinical stage I-II MCC to accurately stage the disease and guide adjuvant therapy decisions.

The NCCN v2.2026 treatment algorithm for MCC reflects a multimodal approach that integrates surgery, sentinel lymph node biopsy, radiation therapy, and immunotherapy. A key update in v2.2026 is the inclusion of neoadjuvant immunotherapy (nivolumab) as an option for resectable MCC, based on emerging clinical trial data demonstrating pathologic complete response rates exceeding 50% in some series. The standard surgical approach remains wide excision with 1-2 cm clinical margins to investing fascia, or Mohs micrographic surgery / peripheral and deep en face margin assessment (PDEMA). SLNB is recommended for all clinical stage I-II patients, ideally performed at the time of definitive wide excision or coordinated with Mohs excision. Adjuvant radiation therapy plays a particularly strong role in MCC management. RT to the primary site and/or draining nodal basin is recommended for most patients, given the high locoregional recurrence rate of MCC. For node-positive disease, completion lymph node dissection or definitive RT to the nodal basin is indicated.

NCCN v2.2026 recognizes Mohs micrographic surgery (specifically PDEMA. Peripheral and deep en face margin assessment) as an appropriate surgical approach for MCC, equivalent to wide excision with 1-2 cm margins. Mohs/PDEMA provides the advantage of complete margin assessment, which is valuable in MCC given the potential for subclinical extension, particularly in anatomically constrained sites such as the head and neck where wide 1-2 cm margins may compromise critical structures. Studies have demonstrated that Mohs for MCC achieves local recurrence rates comparable to wide excision. However, the use of Mohs in MCC carries unique considerations that differ from its application in BCC or SCC.

Immunosuppression is one of the strongest risk factors for MCC development, aggressive behavior, and poor outcomes. Solid organ transplant recipients have an approximately 15-fold increased risk of developing MCC compared to the general population, and their tumors tend to present at a younger age, behave more aggressively, and carry a higher mortality rate. Patients with chronic lymphocytic leukemia (CLL) have an approximately 30-fold increased risk, likely related to both immune dysfunction and shared etiologic factors. HIV/AIDS patients are also at significantly elevated risk. When managing MCC in immunosuppressed patients, several modifications to the standard approach should be considered. In transplant recipients, discuss with the transplant team the possibility of reducing immunosuppressive medications. Conversion from calcineurin inhibitors to mTOR inhibitors (sirolimus/everolimus) has shown potential antitumor benefit in some studies. Closer follow-up intervals are warranted, with examinations every 3 months for the first 2 years. Immunotherapy with checkpoint inhibitors must be used with extreme caution in transplant recipients due to the risk of graft rejection, and decisions should involve the transplant team.

Systemic therapy is indicated for patients with unresectable locoregional recurrence, in-transit metastases, or distant metastatic disease (stage IV). The introduction of immune checkpoint inhibitors has transformed the treatment of advanced MCC. First-line systemic therapy consists of anti-PD-1/PD-L1 checkpoint inhibitors: pembrolizumab (anti-PD-1) and avelumab (anti-PD-L1) are both FDA-approved for metastatic MCC and recommended as first-line agents by NCCN. Response rates vary by line of therapy: first-line pembrolizumab achieves an ORR of approximately 56%, while second-line avelumab (in chemotherapy-refractory disease) achieves an ORR of approximately 33%, with durable responses observed in a substantial proportion of responders in both settings. MCPyV-positive tumors and UV-associated (virus-negative) tumors with high mutational burden both respond well to immunotherapy, though through different mechanisms. Viral antigen presentation versus neoantigen load, respectively. For patients who progress on checkpoint inhibitors or who are not candidates for immunotherapy (e.g., transplant recipients), second-line chemotherapy with platinum-based regimens (cisplatin or carboplatin plus etoposide) can be considered. Chemotherapy produces response rates of 50-60% but responses are typically short-lived (median duration 3-8 months) and carry significant toxicity. Novel approaches under investigation include combination immunotherapy, adoptive T-cell therapy, and targeted agents.

MCC carries a more aggressive prognosis than most cutaneous malignancies, with overall 5-year survival rates that vary significantly by stage. Localized disease (stage I-II) has a 5-year overall survival of approximately 50-65%, node-positive disease (stage III) approximately 35-40%, and distant metastatic disease (stage IV) approximately 14-18%. The majority of recurrences occur within the first 2-3 years after diagnosis, with approximately 40% of patients developing recurrence. Locoregional recurrence is most common, followed by distant metastasis to liver, bone, brain, lung, and skin. Given this recurrence pattern, NCCN recommends intensive surveillance with complete skin and lymph node examination every 3-6 months for the first 3 years after diagnosis, then every 6-12 months thereafter. Imaging (CT or PET/CT) should be considered for patients with high-risk features, node-positive disease, or those who have completed treatment for advanced disease. Patients should be educated about the signs of recurrence and the importance of adherence to the surveillance schedule.