Expression of TTF-1 in breast cancer independently of ER expression: A case report and pathogenic implications

Thyroid Transcription Factor 1 (TTF-1) is often used for the immunohistochemical evaluation of lung cancers, as a significant proportion of these cancers are positive while other adenocarcinomas are usually negative. Breast cancers are known to express TTF-1 only in a small minority of cases but this may be problematic when this staining is used for its differential diagnosis from lung cancer. We present a case of ER-positive breast cancer in 30% of tumor cells that was also TTF-1 positive in some areas of the primary tumor but lost ER expression completely in the metastatic recurrence site while retaining TTF-1 positivity. Additionally, a PTEN mutation was present on genomic evaluation of the primary tumor. Diagnostic, pathogenic and therapeutic implications of these findings are discussed.