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Case 7 - Discussion

Surgical Pathology Case


CK20, 4

CK20, 40


Merkel Cell Carcinoma

The Key Features

  • Small blue round uniform tumor cells

  • CK20, perinuclear globule pattern

  • Most cases positive for Merkel cell polyoma virus

Clinical Futures

  • Also named primary neuroendocrine carcinoma of the skin

  • Predilection for the head and neck of older adults, sun-exposed area

  • Distant metastasis in 40% patients

Gross Findings

  • Pink-red or violaceous, firm, dome-shaped solitary rapidly growing nodule.

Microscopic Findings

  • Sheet, nested, or trabecular growth pattern

  • Monotonously uniform, small, round to oval; finely dispersed chromatin; nucleoli not prominent; Scant cytoplasm and faintly amphophilic

  • Abundant mitoses; Extensive necrosis and individual cell necrosis

  • Delicate fibrovascular stroma

Differential Diagnosis



Pan CK CK20 CK5/6 P63 TTF1 CD99 CEA S100




Merkel cell carcinoma + + +/−
Metastatic small cell carcinoma (lung) + + +/− +/−
Small cell squamous carcinoma + + + +/−
Small cell eccrine carcinoma + +/− +/− +/−
Small cell melanoma +/− + + +
Primitive neuroectodermal tumor +/− + +
Non-Hodgkin's Lymphoma +/− +/− +

Immunohistochemistry Straining

  • Positive: CK20, NSE, chromogranin, synaptophysin, somatostatin, calcitonin, VIP, CD117

  • Most cases also positive for PAX5 and TdT, although often focal and weak

  • Most cases positive for Merkel cell polyoma virus

  • Negative: S-100, CEA, TTF1

Election Microscopy

  • Paranuclear whorls of intermediate filaments

  • Membrane-bound dense-core secretory granules

Treatment and Prognosis

  • Very aggressive tumor


  • AFIP, non-melanocytic tumors of the skin, series 4