[548] Merkel Cell Carcinoma Immunoreactivity with Pax-5

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

Background: Merkel cell carcinoma (MCC) is an uncommon high-grade neuroendocrine carcinoma of the skin. The differential diagnosis includes tumors with small round blue cell morphology. Pax-5 is a B cell specific transcription factor that plays an important role in B cell ontogeny and is commonly expressed in B cell non-Hodgkin's lymphoma, Hodgkin's lymphoma, alveolar rhabdomyosarcoma, and small cell carcinomas. The purpose of this study is to characterize the expression of Pax-5, using a monoclonal antibody of a clone SP34 different from those adopted in previous reports, and a panel of immunohistochemical markers in MCC patients.
Design: We used antibodies against Pax-5, terminal deoxynucleotidyl transferase (TdT), cytokeratin (CK) 7, CK20, thyroid transcription factor 1 (TTF-1), chromogranin and synaptophysin. Immunostaining was recorded semiquantitatively. Pax-5 nuclear staining was evaluated using a 3-tiered scale (weak, moderate, strong) and Pax-5 reactivity as follows: reactivity in fewer than 1% of tumor cells was scored as negative, reactivity in 1% to 25% tumor cells as 1+, reactivity in 26% to 50% tumor cells as 2+, and reactivity in more than 50% of tumor cells as 3+.
Results: Fifty-one MCC cases were reviewed. The cases occurred in 37 males and 14 females, ranging in age from 49 to 95 years (mean 75.9). Fourteen of the lesions were from the face, 10 from the trunk, 3 from the upper limbs, 17 from the lower limbs and 7 were metastases. Of 51 MCC cases, 34 (67%) were positive for Pax-5, showing, 1+ in 13/51 (25%), 2+ in 10/51 (20%), and 3+ in 11/51 (22%) tumor cells reactive. Nuclear staining intensity was weak in 20/51 (39%) and moderate in 14/51 (28%) MCC cases. No MCC (0%) demonstrated strong nuclear staining intensity for Pax-5. The immunohistochemical profile of 22 MCCs was further analyzed and demonstrated expression of TdT in 14/22 (64%), CK20 in 20/22 (91%), chromogranin in 16/22 (73%), and synaptophysin in 22/22 (100%). No MCC expressed CK7 (0%) or TTF-1 (0%). Of the 2 CK20 negative MCC cases, 1 case was positive for TdT, while both were positive for Pax-5.
Conclusions: The results indicate that Pax-5 is frequently expressed in MCC, showing on average 26% to 50% tumor cells reactive and moderate to weak nuclear staining intensity. Pax-5 may also be beneficial in rare cases of CK20-/TdT+ and CK20-/TdT- MCC. Pax-5 positivity in conjunction with CK20, TdT and neuroendocrine markers further supports the diagnosis of MCC when evaluating cutaneous small round blue cell tumors.
Category: Dermatopathology

Tuesday, March 20, 2012 9:30 AM

Poster Session III # 116, Tuesday Morning


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