Thyroid transcription factor‐1 immunohistochemistry: diagnostic tool and malignancy marker in canine malignant lung tumours

Distinguishing primary lung carcinomas (PLCs) from metastases is a challenging task. The diagnostic and prognostic relevance of thyroid transcription factor-1 (TTF-1), a nuclear protein expressed in follicular cells of the thyroid gland and pneumocytes, was tested in 34 primary and 27 nonprimary canine lung tumours. Normal pneumocytes stained negatively in 14 PLCs because of overfixation or prolonged storage of paraffin blocks and were excluded from the study. Among the 20 immunoreactive PLCs, 17 showed strong nuclear positivity. The three tumours that scored negative were two squamous cell and one papillary carcinoma. Metastatic tumours were always negative. TTF-1 was 100% specific and 85% sensitive for PLCs. There was no significant relationship among the percentage of labelled tumour cells (TTF-1 index) and the considered clinicopathological parameters (age, gender, histological type, tumour grade, TNM stage, node status and MIB-1 index). TTF-1 immunohistochemistry may give useful additional information regarding the origin of canine lung tumours, whereas its prognostic use still needs to be determined.