Bizarre lymphoid cells in serous effusion of a dog with mediastinal lymphoma.

An 8-year-old collie dog was examined because of dyspnoea. Thoracocentesis revealed a modified transudate containing a predominance of large (10 to 15 microns diameter) unusual-appearing mononuclear cells showing cloverleaf-shaped nuclei. These cells were categorized as lymphoid in origin on the basis of ultrastructural and cytochemical methods. Subsequent necropsy examination revealed lymphoma, localized primarily to the cranial mediastinum, but also infiltrating tissues surrounding the thyroid, parathyroid and adrenal glands.