Background: The occurrence of neoplasms in horses is relatively low. Granular cell tumor is a seldom diagnosed neoplasm, usually benign, of mesenchymal origin. Controversies exist regarding its origin, which is possibly from Schwann cells or cells with neuroendocrine differentiation. Despite being one of the main primary neoplasms in the lungs of horses, the number of cases is low in comparison to that of secondary lung tumors. Thus, this study proposes to report the anatomopathological aspects in a horse with granular cell tumor of primary pulmonary origin. Case: An 11-year-old female Quarter Horse breed underwent exploratory right lateral thoracotomy after presenting with chronic respiratory changes. During the operation, tumor masses were found in the right and left caudal pulmonary lobes. Due to the severity of clinical respiratory signs and the extent of the lesions, the animal was subjected to euthanasia and anatomopathological examination. Upon necroscopic examination, a tumor mass was found in the middle third of the left caudal lobe, rounded to flattened, measuring 10.0 × 8.0 cm in height and length, white in color, of firm consistency, smooth and regular surface and rising to the lung surface. When sectioned, the mass showed to be composed of multiple firm and dense circular lobes, separated by a thin layer of connective tissue. The tumor invaded the lumen of nearby segmental and subsegmental bronchi, which were partially or totally obstructed by the mass. In the right lung, multiple similar nodules were observed, accompanied by peritumoral hemorrhage. Histopathological analysis of the new formation revealed a dense cluster of cells that expanded over the lung parenchyma. The neoplastic cells were pleomorphic, moderately cohesive, without defined borders, with abundant cytoplasm, densely eosinophilic and finely granular. Intracytoplasmic granules were well evidenced by periodic acid Schiff staining (PAS). The cell nucleus was rounded to oval, excentric, markedly basophilic and with dense chromatin. There was moderate anisocytosis and mild anisokaryosis, with rare mitotic figures. Immunohistochemical analysis revealed positive staining for anti-vimentin and anti-S100 antibodies, confirming the diagnosis of granular cell tumor.Discussion: Granular cell tumors have no predisposition as to breed, sex or age. However, most of the described cases are reported in female horses aged around 13 years. The advanced age of the diagnosed animals may be related to late definitive diagnosis, since the clinical signs are nonspecific and treated palliatively like other respiratory diseases. Macroscopically, this tumor is more common in the multinodular form and, as observed in this case, it has a greater capacity for infiltration. Histologically, the visualization of large, polygonal cells, with a wide cytoplasm containing eosinophilic granules leads to the diagnosis of granular cell tumor. However, PAS staining and immunohistochemical tests were essential for the diagnostic conclusion in this report, confirming the presence of cytoplasmic granules and the mesenchymal and neuroectodermal origin of this neoplasm, respectively. Thus, considering the low occurrence of pulmonary granular cell tumor, the description of this case contributes to the basis of the knowledge of medical-veterinary professionals about this tumor in its clinical and diagnostic aspects.Keywords: Schwann cells, immunohistochemistry, neoplasia, lung.Descritores: células de Schwann, imuno-histoquímica, neoplasia, pulmão.Título: Tumor de células granulares pulmonar em um equino
