Immunohistochemistry for thyroid transcription factor-1 (TTF-1) is considered an important ancillary technique for distinguishing primary pulmonary adenocarcinomas from metastases. A case of metastatic colonic adenocarcinoma in a bronchial biopsy specimen that showed focal positive nuclear staining for TTF-1 is reported here. The primary colonic adenocarcinoma was retrieved from the file and immunostaining also showed focal nuclear positivity for TTF-1. This confirmed the morphological and clinical impression of colonic metastases in the bronchial biopsy specimen. To investigate this apparent aberrant TTF-1 expression, further sections from primary colonic adenocarcinoma were immunostained for TTF-1 using the SPT24 antibody clone. Three of the six cases studied showed focal nuclear positivity with the SPT24 clone. All six cases were subsequently shown to be negative with the 8G7G1 clone. It is concluded that the SPT24 antibody to TTF-1 may show focal nuclear positivity in adenocarcinomas of colorectal origin such that focal staining in small biopsy specimens should be interpreted with caution.
