Napsin A Expression in Anaplastic Lymphoma Kinase-positive Diffuse Large B-Cell Lymphoma: A Diagnostic Pitfall

Napsin A is frequently used to classify a tumor of unknown origin as lung primary. Recent studies have shown that Napsin A positivity occurs in adenocarcinomas of nonpulmonary origin such as renal cell carcinomas, endometrial carcinomas, and clear cell carcinomas of ovary. Nonspecific reactivity has been reported with polyclonal Napsin A antibody. On the basis of an index case of anaplastic lymphoma kinase-positive diffuse large B-cell lymphoma (ALK-DLBCL) expressing Napsin A, which was misdiagnosed as poorly differentiated carcinoma of pulmonary origin, we studied Napsin A expression in our archived cases of ALK-DLBCL.

A total of 3 cases of ALK-DLBCL were studied for Napsin A immunohistochemistry along with typical immunophenotypic profile of these cases. Archived paraffin-embedded tissue blocks and cytology aspiration smears were used for morphologic interpretation and immunohistochemistry. Rabbit monoclonal Napsin A antibody has been used.

All 3 cases were positive for Napsin A exhibiting strong cytoplasmic positivity. To the best of our knowledge, expression of monoclonal Napsin A in lymphomas has never been reported. ALK-DLBCL should be considered in the differential diagnosis when evaluating a Napsin A-positive tumor of poorly differentiated morphology and of unknown primary. As Napsin A has not been described in the lymphoid tissue development, the significance of Napsin A positivity in hematolymphoid neoplasms is unknown and warrants further investigation.