Merkel Cell Carcinoma – MCC
Merkel cell carcinoma (MCC) is a very rare malignancy, but among the most aggressive of the non-melanoma skin cancers. It arises from malignant transformation of Merkel cells, the neuroendocrine sensory receptor cells of the basal epidermis, or from totipotent stem cells with neuroendocrine features. MCC grows explosively, invades surrounding tissue aggressively and carries a very high risk of spread to regional lymph nodes and to distant sites (lungs, liver, bone, brain), even at an early stage. Its mortality rate is about twice that of melanoma.
At Haute-Derma, our MCC management follows rigorous evidence-based standards: detailed dermoscopic and microscopic diagnosis, sentinel lymph node biopsy (SLNB), definitive surgery with microscopic margin control, and close collaboration with oncology, radiation oncology and immunotherapy teams to give each patient the best possible chance of controlling the disease.
Clinical Features and Pathology in Depth
Pathogenesis and Merkel Cell Polyomavirus (MCPyV)
- Viral pathway (MCPyV-positive): the main cause (about 80% of MCC cases) is random integration of Merkel cell polyomavirus (MCPyV) into the host cell's DNA. The virus then expresses oncogenic T-antigen proteins (large T and small T antigens) that inactivate the tumor suppressor genes p53 and pRb.
- UV pathway (MCPyV-negative): the remaining ~20% of cases are unrelated to the virus. They result from a heavy build-up of genetic mutations caused by ultraviolet (UV) radiation and carry a very high tumor mutational burden (TMB).
- AEIOU clinical clues:
- A (Asymptomatic): no pain, no itch, no symptoms.
- E (Expanding rapidly): very fast growth, doubling in size within a few weeks.
- I (Immune suppressed): common in immunosuppressed patients (organ transplant recipients, HIV, blood cancers).
- O (Older than 50): patients over 50, especially over 70.
- U (UV-exposed location): sun-exposed skin (the head and neck account for 50%).
Histopathology
- Small round blue cell tumor: the tumor sits in the dermis and subcutis and is made up of nests and sheets of round to oval neuroendocrine epithelial cells with very scant cytoplasm, large nuclei and finely dispersed "salt-and-pepper" chromatin.
- Very high mitotic activity: numerous abnormal mitoses and apoptotic bodies are scattered throughout the tumor.
Differential Diagnosis and Immunohistochemistry (IHC)
Because it belongs to the "small round blue cell tumor" group, MCC is easily mistaken for metastatic small cell lung cancer (SCLC), small-cell melanoma, lymphoma or poorly differentiated squamous cell carcinoma. An immunohistochemistry (IHC) panel is therefore essential:
| Condition | Distinguishing Microscopic and IHC Features |
|---|---|
| Merkel cell carcinoma (MCC) | CK20 (+) with the characteristic perinuclear dot-like pattern; chromogranin A (+); synaptophysin (+); INSM1 (+); TTF-1 (–); LCA (–). |
| Metastatic small cell lung cancer (SCLC) | TTF-1 strongly (+); CK20 (–); neuroendocrine markers (+). |
| Melanoma (small-cell variant) | S100 (+); SOX10 (+); Melan-A (+); CK20 (–). |
| Lymphoma | LCA/CD45 (+); cytokeratins (–). |
Our Approach to Merkel Cell Carcinoma
Given how aggressive MCC is, Haute-Derma uses a multidisciplinary team (MDT) strategy:
- Urgent biopsy and microscopic assessment: an excisional or deep punch biopsy with a full IHC panel as soon as MCC is clinically suspected.
- Whole-body staging with PET/CT: patients with confirmed MCC should have whole-body PET/CT to look for nodal and distant spread before treatment is planned.
- Sentinel lymph node biopsy (SLNB) as standard: up to 30% of patients have hidden (occult) nodal metastases even when the nodes feel normal, so SLNB is a required step.
- Personalized multimodal treatment: definitive surgery, wide-field adjuvant radiotherapy and modern immunotherapy (checkpoint inhibitors).
How We Diagnose Merkel Cell Carcinoma at Haute-Derma
To avoid losing precious time and to stage the disease accurately, Haute-Derma follows a detailed four-step process:
- Clinical exam and AEIOU checklist: we look for a firm, shiny, painless red-purple (violaceous) nodule that has grown quickly on sun-exposed skin.
- Deep tissue biopsy (punch or core biopsy): a sample deep enough to include the dermis and subcutis so the small round blue cell pattern can be seen.
- Confirmatory IHC panel: CK20, synaptophysin, chromogranin A, TTF-1 and S100 staining to confirm the dot-like CK20 pattern and rule out metastatic lung cancer.
- Whole-body imaging and nodal assessment (PET/CT and SLNB): PET/CT to look for secondary lesions, with sentinel node mapping and biopsy carried out together with a nuclear medicine center.
How We Treat It at Haute-Derma
MCC treatment is tailored to each patient according to the AJCC stage:
- Wide local excision (WLE):
- The tumor is removed with 1.0–2.0 cm clinical margins, down to the deep fascia.
- Alternatively, Mohs micrographic surgery (MMS) is used at delicate facial sites to achieve clear (R0) margins while preserving important anatomy.
- Lymph node management:
- If the SLNB is negative: adjuvant radiotherapy to the nodal basin is still considered for high-risk cases.
- If the SLNB is positive or nodes are clinically enlarged: regional lymph node dissection (lymphadenectomy) combined with adjuvant radiotherapy.
- Adjuvant radiotherapy:
- MCC is highly radiosensitive. Wide-field radiotherapy to the tumor bed and draining lymph nodes is recommended after surgery in most cases to minimize the risk of local recurrence.
- Systemic immunotherapy (immune checkpoint inhibitors, ICIs):
- The new gold standard for locally advanced or metastatic MCC: anti-PD-1/PD-L1 drugs such as avelumab, pembrolizumab or nivolumab. These agents have transformed the outlook for MCC, with high and durable response rates.
