We report a case of prostatic ductal adenocarcinoma which expressed thyroid transcription factor-1 (TTF-1) on immunohistochemical examination, and highlight the fact that this aberrant staining is a potential pitfall in the evaluation of this lesion. Prostatic core biopsy specimens from a 65-year-old man revealed an adenocarcinoma with large-calibre glands lined by stratified hyperchromatic nuclei. There was no accompanying conventional acinar adenocarcinoma and TTF-1 was distinctly positive in the nuclei of the malignant cells. This led to a search for possible lung and other primaries. When a thorough systemic investigation disclosed no evidence of other primary tumours, a radical prostatectomy was performed which revealed predominant ductal adenocarcinoma with a minor component of acinar adenoarcinoma. TTF-1 measurement was repeated which showed staining confined to the ductal adenocarcinoma. Prostate specific antigen (PSA), PSAP and racemase were also found. Our findings underscore the increasing spectrum of lesions that may express TTF-1 on immunohistochemical examination.

TTF-1 is part of a family of homeodomain transcription factors with restricted expression in the thyroid and lung.^1–3 Its immunohistochemical detection has been used in diagnostic surgical pathology to distinguish thyroid and lung tumours from those of other organs.^4,5 Specifically, antibodies to TTF-1 have been reported to be useful in distinguishing pulmonary adenocarcinoma from other primary carcinomas.⁶ TTF-1 is expressed by differentiated thyroid neoplasms.⁷ It can assist in the differential diagnostic investigation of mesothelioma from pulmonary adenocarcinoma, small cell lung carcinoma from Merkel cell carcinoma,^2,3 and neuroendocrine tumours of the lung from well-differentiated neuro-endocrine tumours at other sites.^1,8 TTF-1 mRNA has been discovered in orbital tissues,⁹ C cells and parathyroid cells.¹⁰

More recently, nuclear TTF-1 protein expression was reported in primary and metastatic colonic adenocarcinoma,^11,12 primary ovarian epithelial neoplasms,¹³ thyroid-like nasopharyngeal papillary adenocarcinoma,⁷ gastric …