Diagnostic challenge of pulmonary metastatic transmissible venereal tumour in a young dog.

Canine transmissible venereal tumour (TVT) is a clonally transmissible round cell neoplasm that most commonly affects the external genitalia. Extragenital and metastatic forms are uncommon and may represent a substantial diagnostic challenge, particularly when genital lesions are absent. We describe a young dog presenting with large unilateral pulmonary masses that clinically and histologically mimicked lymphoma. Histopathology revealed a malignant round cell neoplasm composed of sheets of poorly differentiated round cells. Immunohistochemical evaluation showed diffuse cytoplasmic immunolabelling for vimentin and neuron-specific enolase, while markers of epithelial, neuroendocrine and definitive lymphoid differentiation were negative. Weak cytoplasmic CD79a immunolabelling in a subset of neoplastic cells further complicated interpretation. Despite extensive immunohistochemical profiling and consultation among multiple board-certified pathologists, a definitive diagnosis could not initially be reached. Following recognition of the dog's epidemiological background as an imported stray from Romania, TVT was considered and subsequently confirmed by detection of the characteristic LINE-MYC oncogene rearrangement using polymerase chain reaction. This case highlights an important diagnostic pitfall in pulmonary round cell tumours and emphasises the value of integrating epidemiological information, histopathology, immunohistochemistry and molecular diagnostics when evaluating atypical extragenital presentations of TVT.