Cancer of unknown primary in a mare: case report and comparative pathology review

Abstract

A 25-y-old Percheron mare was admitted to the teaching hospital because of lethargy and intractable dyspnea. Thoracoabdominal ultrasound examination identified severe peritoneal effusion, mild bilateral pleural effusion, and a diffuse pulmonary nodular pattern. Cytology of peritoneal fluid revealed a hypercellular sample with clusters of neoplastic polygonal cells and admixed macrophages. Euthanasia was followed by postmortem examination; marked bi-cavitary effusion was present, and innumerable up to 4-cm diameter, round-to-floriform nodules were diffusely evident throughout serosal surfaces as well as the pulmonary and hepatic parenchyma. Disseminated adenocarcinoma, predominantly affecting lung and liver with widespread serosal implantation, was confirmed on light microscopy. Neoplastic cells had strong immunolabeling for pancytokeratin and lacked immunoreactivity to vimentin, napsin A, and Pax8. Cytokeratin 7 and thyroid transcription factor-1 were non-contributory given absent and inconsistent internal control reactivity, respectively. Such results, combined with the lack of a major mass that would indicate a primary site, were supportive of carcinoma of unknown primary site, which remains a conundrum in human oncology, and is poorly explored in veterinary medicine, mainly as a result of clinical and diagnostic limitations.

Keywords: adenocarcinoma; cancer; carcinomatosis; horses; immunohistochemistry; unknown primary.

Figure 1.

Cytopathology of peritoneal fluid of a mare containing clusters of round-to-polygonal neoplastic cells with variable amounts of vacuolated cytoplasm. Anisocytosis and anisokaryosis are moderate with some multinucleation (arrow). Erythrocyte- and hemosiderin-laden macrophages are scattered throughout (arrowhead). Romanowsky stain.

Figure 2.

Adenocarcinoma of unknown primary site in a mare. A. Bi-cavitary macroscopy with countless nodules and masses covering mesothelial surfaces and organs, most remarkably lung and liver. B. Liver and diaphragm with multiple coalescent, up to 4-cm diameter, tan, firm nodules that extend into the parenchyma (inset).

Figure 3.

Adenocarcinoma of unknown primary site in a mare. A. Neoplastic tubulo-acinar units efface and infiltrate the hepatic parenchyma. H&E. B. The lung is similarly effaced by erratic tubules without luminal secretory material. H&E. C. Neoplastic cells have strong membranous and cytoplasmic immunolabeling (left of image), as does the neoplastic embolus within a portal vein (arrow), and the non-affected biliary duct epithelium (internal positive controls; arrowheads). Pancytokeratin IHC. D. Neoplastic cells lack expression for napsin A (left of image); type II pneumocytes of the adjacent compressed non-affected tissue have strong cytoplasmic reactivity (right of image). Napsin A IHC.