Background: Thyroid transcription factor-1 (TTF-1) is a useful marker for identifying thyroid and lung cancers in diagnostic pathology, particularly for the investigation of unknown primary cancers. However, some other tumors such as colorectal cancer might aberrantly express TTF-1, particularly with the less specific clone SPT24. Occasional diffuse large B-cell lymphoma (DLBCL) cases have been reported to be TTF-1-positive, yet there is no information on TTF-1 expression in peripheral T-cell lymphoma (PTCL). Methods: We investigated a series of PTCL and DLBCL by immunohistochemistry for TTF-1 expression using 2 commercially available clones. Results: We found that 33% (5/15) adult T-cell leukemia/lymphomas (ATLLs) and 25% (2/8) angioimmunoblastic T-cell lymphomas (AITLs) were positive by clone SPT24 and only 2ATLL cases were positive by clone 8G7G3/1. Overall TTF-1 expression rates of PTCL by SPT24 and 8G7G3/1 were 16% (7/43) and 5% (2/43), respectively. All DLBCLs were negative. Conclusion: Although TTF-1 is a relatively specific marker for thyroid and lung cancers, it might be expressed in some lymphomas, particularly PTCL when using clone SPT24. Pathologist should be aware of this possible diagnostic pitfall when using TTF-1 in investigating tumors of unknown origin.
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