A 3-year-old Labrador retriever dog was presented with pyrexia, dyspnoea and tachycardia. A pleural effusion was detected radiographically and ultrasonography showed pleural fluid with floating material. The fluid was drained, revealing a soft tissue mass adjacent to the left ventricle. The aspirated fluid had a proteinaceous and gelatinous appearance. Cytological examination revealed atypical mesenchymal cells in a dense eosinophilic background, interpreted as consistent with the presence of a matrix-secreting tumour, probably a myxosarcoma. Thoracoscopy confirmed the presence of the mass adjacent to the left ventricle, but showed additional smaller pleural masses. Microscopical and immunohistochemical evaluation of a biopsy sample from the mass supported the diagnosis of a myxosarcoma, which was further confirmed by transmission electron microscopy.
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