Thyroid transcription factor–1 staining is useful in identifying brain metastases of pulmonary origin

Metastatic carcinomas of the brain are a common intracranial tumor and may be the first manifestation of the cancer in a patient. The primary site of these tumors is often difficult to determine based solely on histology. Thyroid transcription factor–1 (TTF-1) is a fairly specific immunohistochemical marker for primary lung and thyroid carcinomas. Recent studies have indicated that TTF-1 may be useful in identifying metastatic brain carcinomas arising from primary lung carcinomas. This study evaluates TTF-1 immunoreactivity in 101 metastatic brain carcinomas and 50 glioblastoma multiform. In patients with a known pulmonary primary, 59% of the metastases were positive for TTF-1. The one patient with a thyroid primary also had a metastasis positive for TTF-1. None of the metastatic lesions arising from other sites was positive for TTF-1. None of the 50 glioblastoma multiform was positive for TTF-1. This study indicates that TTF-1 may be useful in evaluating metastatic carcinomas of unknown primary and TTF-1 immunoreactivity may be indicative of a lung or thyroid primary.