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Int J Clin Exp Pathol 2013;6(2):142-147

Original Article
Immunohistochemical expression of PAX5 and TdT by
Merkel cell carcinoma and pulmonary small cell
carcinoma: a potential diagnostic pitfall but
useful discriminatory marker

Ravindra Kolhe, Michelle D Reid, Jeffrey R Lee, Cynthia Cohen, Preetha Ramalingam

Department of Pathology, Georgia Health Sciences University, Augusta, GA; Department of Pathology, Emory
University School of Medicine, Atlanta, GA; Department of Pathology, Charlie Norwood Veterans Affairs Medical
Center, Augusta, GA, United States; The University of Texas MD Anderson Cancer Center, Houston, TX, USA.

Received November 26, 2012; Accepted December 15, 2012; Epub January 15, 2013; Published February 1,
2013

Abstract: Background: Merkel cell carcinoma is a high-grade neuroendocrine carcinoma of skin that is characterized
by immature cells which, because of its striking morphologic similarity, may be confused with other small round
blue cell tumors such as pulmonary small cell carcinoma or lymphoblastic leukemia/lymphoma. Immunohistochemistry
is therefore paramount to ensuring accurate diagnostic distinction between these tumors. The aim of our study
was to evaluate and compare the expression of PAX5 and Terminal deoxynucleotidyl transferase (TdT), in Merkel
cell carcinoma and pulmonary small cell carcinoma. Design: PAX5 and TdT immunohistochemical stains were performed
on 27 Merkel cell carcinomas and 10 pulmonary small cell carcinomas. Results: PAX5 was expressed in
24/27 (89%) Merkel cell carcinomas and 0/10 (0%) pulmonary small cell carcinomas. TdT was expressed in 21/27
(78%) Merkel cell carcinomas and 9/10 (90%) pulmonary small cell carcinomas. Conclusions: Our study confirms
that PAX5 and TdT expression can be expressed in a high percentage of Merkel cell carcinomas and so when positive
are not diagnostic of lymphoblastic leukemia/lymphoma. When dealing with metastatic lesions, PAX5 negativity
would favor a diagnosis of pulmonary small cell carcinoma over Merkel cell carcinoma. In addition, TTF-1 negative
pulmonary small cell carcinoma is to be differentiated from Merkel cell carcinoma. (IJCP1211035).

Keywords: Merkel cell carcinoma, PAX5, TdT, small cell carcinoma, lung

Address all correspondence to:
Dr. Preetha Ramalingam
Department of Pathology
The University of Texas MD Anderson Cancer Center
1515 Holcombe Blvd, Box 85, Houston, TX 77030, USA.
Phone: 713-7945698; Fax: 713-7458610
E-mail: pramalingam@mdanderson.org