Concurrent thoracic mesothelioma and thyroid C-cell adenoma with amyloid deposition in an aged horse

Abstract

A 21-year-old American Saddlebred mare died with a history of weight loss and breathing difficulties of 1 month duration. Post-mortem examination revealed a copious pleural effusion with multifocal to coalescing numerous white to grey nodular masses on the serosal surface of the pericardium, lungs and thoracic cavity. In addition, the left thyroid gland was markedly enlarged. A thoracic mesothelioma and C-cell adenoma with amyloid deposits of the left thyroid gland were diagnosed by histopathology and confirmed by immunohistochemistry employing antibodies against cytokeratin (CK), vimentin and calcitonin. Amyloid deposits in the thyroid tumour were confirmed by Congo red staining with apple-green birefringence under polarized light. Mesothelioma remains an uncommon neoplasm encountered in aged horses. Discussion includes the diagnostic challenge of differentiating carcinomatosis from mesothelioma by histology and differentiating reactive and neoplastic mesothelial cells by cytology.

Keywords: Aged horse; Amyloid; C‐cell adenoma; Equine; Mesothelioma; Thyroid adenoma.

Figure 1

Pathological aspects of the thoracic mesothelioma and C‐cell adenoma in an aged horse. (a) Multifocal to coalescing masses are easily identified on the pleural surfaces and thoracic wall. (b) The left thyroid gland is replaced by an expansile neoplasm contained within the pre‐existing thyroid capsule.

Figure 2

Photomicrographs of the mesothelioma (a, b) C‐cell adenoma (c, d). (a) The serosal surface is infiltrated and replaced by an extensive proliferation of polygonal cells forming nests, cords and ducts of variable size and shape. (b) At higher magnification, neoplastic cells have indistinct cell borders, moderate amounts of cytoplasm, oval nuclei with stippled chromatin and variably distinct nucleoli. (c) The left thyroid mass consists of compact sheets (or solid clusters) of polyhedral cells and compresses the remaining parenchyma at the periphery. (d) At higher magnification, neoplastic cells have moderate amount of eosinophilic faintly granular cytoplasm and round pleomorphic nuclei with euchromatic to stippled chromatin and variably distinct nucleoli.

Figure 3

Immunohistochemical characterization of the mesothelioma. Neoplastic cells express pan‐cytokeratin (a) and vimentin (b).

Figure 4

Immunohistochemical characterization (a) and Congo red staining (b, c) of the left thyroid mass. (a) Positive cytoplasmic labelling for calcitonin in C cells of the thyroid tumour. (b) The hyalinized stroma of the C‐cell adenoma is positive for Congo red stain. (c) The congophilic material exhibits characteristic apple‐green birefringence under polarized light, specific for amyloid.