Surfactant Protein A and Napsin A in the Immunohistochemical Characterization of Canine Pulmonary Carcinomas: Comparison With Thyroid Transcription Factor-1.

Thyroid transcription factor-1 (TTF-1) is a specific and sensitive marker for canine pulmonary tumors but is also expressed in thyroid carcinomas, which commonly metastasize to lung. Napsin A and surfactant protein A (SP-A) are used in the histologic diagnosis of non-small-cell lung cancer in humans but have not been thoroughly evaluated in neoplasms of dogs. The objective of this study was to compare the efficacy of immunohistochemistry for SP-A, napsin A, and TTF-1 in the diagnosis of canine pulmonary carcinomas. TTF-1, napsin A, and SP-A antibodies were applied to 67 formalin-fixed, paraffin-embedded canine pulmonary tumors. Although each marker had good sensitivity, only 3% (2/67) of lung tumors were negative for SP-A compared with 7% (5/67) and 9% (6/67) for napsin A and TTF-1, respectively. Each antigen was detected in a greater percentage of cells of tumors with acinar or papillary patterns compared with those with squamous differentiation. SP-A immunoreactivity was absent in all 113 nonpulmonary tumors tested. Of 108 normal tissues, SP-A was detected only in lung and in 1 of 6 adrenal, 1 of 3 endometrial, and 1 of 4 hepatic sections. Based on these findings, SP-A and napsin A are useful markers of canine lung epithelial neoplasia. Of these, SP-A is the most sensitive and specific (a possible pitfall is the need to distinguish entrapped normal pulmonary epithelial cells or alveolar macrophages from neoplastic cells) and can be used in combination with TTF-1 or napsin A to improve detection and differentiation of pulmonary carcinomas from metastatic tumors in the canine lung.