Mixed mesenchymal tumor in the kidney of a young beagle dog

A rare spontaneous tumor was detected in the kidney of an 11-mo-old beagle dog. The tumor was a well-circumscribed cortical mass and was composed of a heterogeneous population of connective tissue cell types. While the primary cell type was a spindle cell associated with prominent collagen deposition, other areas contained bands of smooth muscle cells and poorly cellular myxomatous tissue. The presence of smooth muscle cells was confirmed by transmission electron microscopy and immunoperoxidase techniques. Based on the results, the tumor was diagnosed as a benign mixed mesenchymal tumor.