Immunohistochemical Detection of Pax8 and Napsin A in Canine Thyroid Tumours: Comparison with Thyroglobulin, Calcitonin and Thyroid Transcription Factor 1.

Expression of thyroid transcription factor (TTF)-1 corroborates a thyroid origin of neoplasms. Thyroglobulin and calcitonin immunohistochemistry (IHC) can distinguish between a follicular and C-cell origin of thyroid tumours, respectively. Pax8 (expressed by normal canine thyroid follicular cells) and napsin A (expressed mainly by C-cells) labelling was compared with labelling for TTF-1, thyroglobulin and calcitonin in 114 canine proliferative thyroid lesions. All 81 follicular tumours expressed thyroglobulin and were negative for calcitonin; 79/81 (98%) of these tumours expressed TTF-1 and Pax8 and 60/81 (74%) expressed napsin A. All 25 C-cell lesions expressed calcitonin and were negative for expression of thyroglobulin; 22 (88%) were positive for TTF-1, 13 (57%) for Pax8 and 24/24 for napsin A. Six mixed follicular-medullary carcinomas expressed all five markers. Both carcinosarcomas expressed TTF-1 and napsin A, and one each of these tumours expressed thyroglobulin, calcitonin or Pax8. Pax8 expression was also detected in epididymal cells, endometrial cells and vas deferens epithelium, in Sertoli-like ovarian cells, and in some cases of ovarian adenoma, pancreatic carcinoma, renal cell carcinoma and Sertoli cell tumour. Napsin A was also detected in adrenocortical cells, ovarian granulosa cells, epididymal and endometrial cells, as well as in some renal cell carcinomas, pulmonary adenocarcinomas and Sertoli cell tumours. In summary, Pax8 was as sensitive as TTF-1 and slightly less sensitive than thyroglobulin for identification of follicular tumours, but had low sensitivity for C-cell tumours. Napsin A was as sensitive as calcitonin for C-cell neoplasms, but was less sensitive than thyroglobulin for follicular neoplasms. Thus, these markers are sensitive and, except for renal cell carcinoma (for Pax8, napsin A) and pulmonary adenocarcinoma (for napsin A), are specific thyroid tumour markers.