Canine polyostotic B-cell lymphoma: a case with clinical, immunohistochemical, and flow cytometric characterization, and review of the literature.

An 8-mo-old Mastiff-cross dog with bone pain and lytic-proliferative lesions in the radius, ulna, femur, vertebral spinous processes, and ribs, was diagnosed with lymphoma. The dog also had anemia and thrombocytopenia, and atypical circulating lymphocytes were identified as B cells by flow cytometry. Due to the multicentric, rapidly progressive disease, the dog was euthanized. Postmortem examination confirmed extensive bone replacement by lymphoma, and infiltration of lymph nodes, spleen, and liver. Histomorphology and immunohistochemistry showed a diffuse large B-cell lymphoma that was immunopositive for PAX5 and CD20, and immunonegative for CD3. Lymphoma of bone is rare in dogs and humans, and is most frequently reported in pediatric individuals. Including our case, 7 of 14 reported cases occurred in dogs <2-y-old, and all but 1 had polyostotic disease. Long bones, ribs, and vertebrae were affected most often, and the distal metaphyseal region was targeted in long bones. Visceral and nodal tissue infiltration was common, and all tumors had a diffuse architecture. Most dogs with polyostotic lymphoma were euthanized at the time of diagnosis, and survival was <6 wk in dogs that were treated with chemotherapy or surgery.