Merkel Cell Carcinoma
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Direct answer
Among primary skin cancers, Merkel cell carcinoma is unusually lethal: a neuroendocrine carcinoma of the Merkel touch receptor that presents as a rapidly enlarging, painless, shiny violaceous nodule on the sun-damaged head and neck or limb of an elderly or immunosuppressed patient. Two aetiological streams converge — ultraviolet damage and clonal integration of Merkel cell polyomavirus in roughly 80 per cent of tumours. Histology shows a dermal small round blue cell tumour, and immunohistochemistry seals the diagnosis with dot-like paranuclear CK20 positivity and neuroendocrine markers, contrasting with the TTF-1 positivity of metastatic small cell lung carcinoma. Sentinel lymph node biopsy is standard at staging because nodal spread is common at presentation, and recurrent or metastatic disease is treated with anti-PD-L1 immunotherapy such as avelumab.
What you must remember
- Cell of origin and behaviour: neuroendocrine (Merkel) cells of the basal epidermis; locally aggressive with high regional nodal and distant metastatic rates, and mortality exceeding many melanoma presentations.
- The AEIOU shorthand: Asymptomatic or non-tender nodule, Expanding rapidly, Immunosuppression, Older than 50, Ultraviolet-exposed fair skin — a presentation checklist worth reciting.
- Two hit-and-run pathways: ultraviolet-driven (mutational signature, on chronically sun-exposed skin, more common in white populations) and Merkel cell polyomavirus-associated (clonal viral integration, driven by T and small T antigens).
- Immunosuppression link: solid-transplant recipients and chronic lymphocytic leukaemia patients have multi-fold increased risk — the Merkel cell — CLL association being specifically quotable.
- Histology: sheets and nests of uniform small blue cells with scant cytoplasm, brisk mitoses and apoptosis in the dermis, extending into subcutis; epidermal involvement (epidermotropism) occurs in a minority.
- The immunohistochemistry that decides: CK20 in characteristic paranuclear dot pattern, positive synaptophysin and chromogranin, negative TTF-1 (excluding pulmonary small cell carcinoma) and negative leukocyte common antigen (excluding lymphoma).
- Management pathway: wide local excision with margins, sentinel lymph node biopsy (upstaging a substantial fraction of clinically node-negative patients), adjuvant radiotherapy to the primary site and nodal basins, and programmed death-ligand 1 blockade for advanced disease.
- Indian context: genuinely rare in darker Indian skin, which is ultraviolet-protected; a fast-growing violaceous nodule in an elderly Indian patient is more often squamous carcinoma or atypical fibroxanthoma — but immunosuppressed patients narrow the gap.
Telling the small blue cells apart
A 70-year-old renal transplant recipient develops a 2-centimetre glossy red-purple nodule on the temple over eight weeks. The differential of a dermal small round blue cell tumour in adults has four residents, and each is excluded by a targeted stain. Cutaneous metastasis of small cell lung carcinoma — excluded by TTF-1 positivity in the pulmonary tumour, since Merkel cell carcinoma is TTF-1 negative. Cutaneous lymphoblastic lymphoma — excluded by negative leukocyte common antigen and T or B cell markers. Merkel cell carcinoma itself — dot-like CK20 with neuroendocrine markers. And a primitive neuroectodermal tumour or cutaneous Ewing sarcoma — CD99 membranous positivity and EWSR1 rearrangement, against Merkel cell carcinoma's negativity.
Staging then decides treatment: the sentinel node, positive in a third or more of clinically node-negative patients, upstages prognosis; a positive node adds completion lymphadenectomy or nodal radiotherapy; the primary site receives wide excision, commonly with adjuvant radiotherapy given the tumour's radiosensitivity and high local recurrence risk. Metastatic disease has become the poster case for checkpoint immunotherapy — avelumab and pembrolizumab produce durable responses in a majority of treated patients, a striking change from the chemotherapy era.
Where the exam sets its traps
The first trap is dismissibility: the nodule is painless and the patient is elderly, so the lesion is watched until it doubles again — the teaching point is that any rapidly growing, non-tender skin nodule in an older or immunosuppressed patient earns a biopsy promptly. The second is the staining pair: candidates invert CK20 and TTF-1. Remember the geography — CK20 marks Merkel cell carcinoma (a skin-primary pattern), TTF-1 marks lung-primary small cell carcinoma; both are small blue cells otherwise indistinguishable on haematoxylin-eosin. Third, do not equate small cell morphology of the skin with lung disease by reflex: cutaneous metastasis is commoner than primary Merkel cell carcinoma in absolute terms, so a chest evaluation accompanies, not replaces, the immunopanel.
Frequently asked questions
Which virus is integrated in most Merkel cell carcinomas?
Merkel cell polyomavirus, clonally integrated in about 80 per cent of tumours, whose T antigens drive proliferation.
What immunohistochemical profile identifies Merkel cell carcinoma?
Dot-like paranuclear CK20 with synaptophysin and chromogranin positivity, and TTF-1 negativity — the pattern that separates it from metastatic small cell lung carcinoma.
Why is sentinel lymph node biopsy performed routinely?
Occult nodal involvement is common at presentation, sentinel status upstages disease and directs nodal therapy, making it standard in the initial management.
Which patients are at specially increased risk?
Older, fair-skinned, chronically sun-exposed individuals and the immunosuppressed — transplant recipients and chronic lymphocytic leukaemia patients in particular.
What is the first-line treatment of advanced Merkel cell carcinoma?
Checkpoint immunotherapy with anti-programmed death-ligand 1 agents such as avelumab, which has largely replaced cytotoxic chemotherapy for metastatic disease.