Myocardial Fibrosis with Cartilaginous Tissue Formation in Right Atrial Auricle of a Dog.

A 13-year-old castrated male Miniature Dachshund exhibited oedema of the face and cervical region. Clinical examination, including computed tomography, revealed a mass on the right atrial wall. Histopathological examination of the mass revealed proliferation of spindle cells, which formed intersecting bundles and abundant proteoglycan material. Multiple islands of hyaline cartilage were also observed within foci of proliferated spindle cells. Although the proliferated spindle cells replaced the myocardial parenchyma, there was no evidence of malignancy. The spindle cells were immunopositive for vimentin and α-smooth muscle actin, while chondrocytes were immunopositive for vimentin and S100. Neither the spindle cells nor the chondrocytes were immunolabelled for cytokeratin AE1/AE3, desmin, factor VIII, melan A, p63 or Ki-67. These findings indicate that the lesion represented myocardial fibrosis with cartilaginous tissue formation.