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Case 73 - Discussion

Hematopathology Case



CD20 CD79a
CD10 CD3
CK20 Synaptophysin
CD117 Chromogranin A
CK AE1/3  
  • Diagnosis: Merkel cell carcinoma (MCC)

  • Microscopic description: The tumor consists of monotonous, small, round and blue cells with very high N/C ratio and evenly dispersed chromatin. The nucleolus is inconspicuous. The tumor cells are dyscohesive with focal vaguely nodular pattern. Abundant mitotic figures are present.

  • Discussion: this patient had no skin lesions. Nodal MCC may occur without no skin primary prior to or after the diagnosis. The exact origin of the nodal lesion is uncertain. Notably, most MCC cases also express PAX5 and TdT, although not uniform and strong in most cases.

  • Immunoprofile: CK20+,  CD117+, Synaptophysin+, chromogranin A+, PAX5 +/-, TdT +/-, CD45-, CK5/6-, p63-, TTF1-, CEA-, S100-, vimentin-

  • Differential diagnosis:

    • T-ALL: young age, mediastinal mass, TdT+, CD3+.

    • B-ALL: young age, more common as leukemia form, TdT+, CD10+, CD20-/+, PAX5+.

    • Metastatic small cell carcinoma for the lung: CK20-, CK5/6+, TTF+ (50%), CEA+/-.

    • Small cell melanoma: S100+, MITF+. MART1+, HMB45+, CK20-.

    • PNET: CD99+, Vimentin+, CK20-.

    • Merkel cell carcinoma:



      PanCK CK20 CK5/6 P63 TTF1 CD99 CEA S100




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