[523] TdT Expression in Rare CK20 Negative Merkel Cell Carcinoma.

Michael Sidiropoulos, Wedad M Hanna, Simon Raphael, Zeina Ghorab. University of Toronto, ON, Canada; Sunnybrook Health Sciences Centre, Toronto, Canada

Background: Merkel cell carcinoma (MCC), also known as primary cutaneous neuroendocrine carcinoma, is an uncommon tumor of the elderly with indistinct clinical features and aggressive behavior. MCCs consist of small round blue cells making the diagnosis of MCC rather challenging and the use of immunohistochemistry (IHC) important. Typically, MCCs are positive for cytokeratin (CK) 20 in a paranuclear dot-like pattern and the neuroendocrine markers synaptophysin and chromogranin, and negative for CK7 and thyroid transcription factor 1 (TTF-1). Recent studies have demonstrated expression of the DNA polymerase terminal deoxynucleotidyl transferase (TdT) in MCCs. Herein we present a case series and immunophenotypic analysis of 5 patients with CK20 negative (CK20-) MCC.
Design: We reviewed a total of 40 MCC cases on file accessioned between 1995-2010 from the archives of Sunnybrook Health Sciences Centre. Only cases that had negative imaging studies with no evidence of a primary lung lesion were included. We identified a set of 5 MCCs with CK20- staining. All 5 cases were analyzed for clinical and pathologic attributes. Two pathologists and one dermatopathologist reviewed all the cases and studied the IHC profile using a panel of antibodies against: CK20, CK7, TdT, TTF-1, chromogranin and synaptophysin. Immunostaining was recorded semiquantitatively.
Results: The cases occurred in three males and two females, ranging in age from 74 to 82 years (mean 78.0). Four out of the 5 (80%) cases were from the lower extremities and the remaining case was a nodal metastasis. TdT staining was positive in 4 out of the 5 cases (80%) showing >25% of tumor cells reactive with moderate nuclear staining intensity. Two and all 5 MCCs showed moderate diffuse positive staining for chromogranin (40%) and synaptophysin (100%), respectively. All 5 cases were negative for CK20, CK7 and TTF-1.
Conclusions: Our study demonstrates that TdT is beneficial in rare cases of CK20- MCC.
Category: Dermatopathology

Wednesday, March 2, 2011 1:00 PM

Poster Session VI # 99, Wednesday Afternoon


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