[495] PAX-5 and TdT Expression in Merkel Cell Carcinoma and Pulmonary Small Cell Carcinoma: A Diagnostic Pitfall but Potentially Useful Discriminatory Marker.

Ravindra B Kolhe, Michelle D Reid, Jeffrey R Lee, Cynthia Cohen, Preetha Ramalingam. Medical College of Georgia, Augusta; Emory University, Atlanta, GA; Charlie Norwood VA Medical Center, Augusta, GA

Background: Merkel cell carcinoma (MCC) is a high-grade neuroendocrine skin carcinoma characterized by cells with scant cytoplasm and fine, even chromatin. It is sometimes difficult to accurately diagnose by light microscopy due to its histologic similarity to other "small round blue cell tumors (SRBCTs)" including lymphoblastic lymphoma and small cell carcinoma. Immunohistochemical stains are often required to make a definitive diagnosis of MCC. PAX-5 is a B-cell specific transcription factor that is crucial for B-cell ontogeny and is detected in most B-cell lymphomas. Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase present in thymic T cells, B lymphoblastic leukemia/lymphoma (B-ALL), and some cases of acute myeloid leukemia. We had a recent case of MCC that expressed PAX-5 and TdT. We therefore sought to evaluate the expression of PAX-5 and TdT in MCC as well as pulmonary small cell carcinoma (PSCC) to determine if they were consistently expressed by these tumors.
Design: Paraffin blocks and slides from 28 MCCs and 10 PSCC were retrieved from the Pathology files of 3 institutions. PAX-5 and TdT immunohistochemical stains were performed and staining was graded as: negative (-), weak (1+), moderate (2+) or strong (3+). Stain distribution was graded as: focal (<10%), patchy (10-50%) or diffuse (>50%).
Results:

Positive Immunohistochemical Staining of PAX-5 and TdT in Merkel Cell Carcinoma and Pulmonary Small Cell Carcinoma

Tumor	PAX-5	TdT
MCC (n=28)	24 (86%)	22 (79%)
PSCC (n=10)	1 (10%)	9 (90%)

MCC: Merkel Cell Carcinoma; PSCC: Pulmonary small cell cacinoma, TdT: Terminal deoxynucleotidyl transferase

PAX-5 was positive in 24/28 (86%) MCC cases while TdT was positive in 22/28 cases (79%) MCCs. In PSCC, PAX-5 was positive in 1/10 cases (10%) and TdT in 9/10 cases (90%).
Conclusions: MCC and PSCC, which are both SRBCTs, coexpress the hematologic lymphoid markers PAX-5 and TdT. PAX-5 was expressed by the majority of MCCs but only rarely by PSCCs. TdT expression was frequent in both PSCC and MCC. Therefore, positivity for TdT and PAX-5 in any small round blue cell tumor should be interpreted with caution to prevent misdiagnosis as B-ALL. In cases where metastatic PSCC is to be differentiated from MCC, and TTF-1 is negative, PAX-5 may be a useful exclusionary stain since its negativity would favor a diagnosis of PSCC over MCC in this setting.
Category: Dermatopathology

Wednesday, March 2, 2011 1:00 PM

Poster Session VI # 100, Wednesday Afternoon