Unusual Expression of Thyroid Transcription Factor 1 and Napsin A in Metastatic Adenoid Cystic Carcinoma of Extrapulmonary Origin in the Lung

Objectives: Our study examines thyroid transcription factor I (TTF-1) expression in 40 primary adenoid cystic carcinomas (ACCs) arising in various sites and compares TTF-1 expression between primary and metastatic ACCs in 12 cases with distant metastases. Methods: Forty patients with ACCs, including 12 pairs of primary and metastatic ACCs, were evaluated for the immunohistochemical expression of TTF-1 (clone SPT24). In addition, 10 metastatic ACCs to the lung were tested on napsin A and a different TTF-1 antibody (clone 8G7G3) for further evaluation. Results: No primary ACCs showed TTF-1 immunoreactivity (clone SPT24). TTF-1 was positive in five (41.7%) of 12 metastatic ACCs; all five cases were found only in the lung and comprised five (50.0%) of 10 cases. In all positive cases, staining was focal and detected only in the cribriform histologic subtype. Staining patterns using both antibodies (both SPT24 and 8G7G3) were very similar, and TTF-1 positive tumor cells were also positive for napsin A. Extrapulmonary ACCs were all negative for TTF-1 regardless of origination and metastasis. Conclusions: TTF-1-and napsin A-positive ACCs in the lung should not be considered primary ACCs because TTF-1 and napsin A can be expressed in metastatic ACCs of the lung.