Verrucoid lesions of mitral valve in a dog with features of inflammatory myofibroblastic tumor

We report a mitral valve lesion detected at autopsy in a 9-year-old male German Shepherd dog suffering from mild mitral regurgitation. Gross examination of the heart showed exophitic, noncontiguous lesions involving the atrial aspect of both mitral leaflets. Microscopic evaluation of the mitral lesions disclosed a diffuse proliferation of myofibroblasts with little atypia, arranged loosely and rather randomly, within a myxoid stroma and associated with inflammatory cells identified as CD138+ plasma cells, CD68+ macrophages, and eosinophils. The myofibroblastic proliferation we describe is quite similar to previously described inflammatory myofibroblastic tumor (IMT), a very rare lesion, exceptionally found in the human heart.