Pathologic and prognostic characteristics of splenomegaly in dogs due to fibrohistiocytic nodules: 98 cases.

Ninety-eight canine splenectomy specimens consisting of combined nodular lymphoid and fibrohistiocytic cell proliferation were evaluated for seven light microscopic characteristics. Electron microscopic features in eight primary and two metastatic nodules (liver) were also evaluated. Nodular fibrohistiocytic proliferation in the canine spleen is characterized by a mixed population of histiocytoid and/or spindle cells in varying proportions intermixed with hematopoietic elements, plasma cells, and/or lymphocytes. These nodules seem to form a continuum between splenic lymphoid nodular hyperplasia and malignant splenic stromal neoplasms (malignant fibrous histiocytoma). Immunohistochemical methods used on 32/98 specimens showed uniform and strong positive staining among fibrohisiocytic cells for vimentin and desmin; S100 protein was similarly stained in general abundance. Individual cells strongly stained with smooth muscle actin were sparse but widely distributed. Proliferating cell nuclear antigen was not useful in the subjective differentiation of nodules taken from dogs that died of spleen-related causes and those surviving 12 months following splenectomy. A spectrum of cell types were observed by electron microscopy within each nodule. Fibroblasts, macrophages, intermediate fibrohistiocytic types, and several forms of splenic reticular cells were present. There were no consistent alterations in hematology or serum chemistry profiles of these dogs to provide useful diagnostic/prognostic information. Among the 93/98 dogs with complete (12 month) follow-up information, 48% (45/93) were alive and 52% (48/93) were dead. Dogs that died or were euthanatized during the follow-up period had a median survival of 5 and 5.5 months, respectively (range 0-15 months). Forty-four percent (21/48) died from causes linked to their splenic disease, and 35% (17/48) died from competing causes. The cause of death in 21% (10/48) was unknown. Lymphoid:fibrohistiocytic proportion and mitotic index in the nodules were anatomic features most predictive of postplenectomy mortality. A higher proportion of lymphoid to fibrohistiocytic type cells was associated with increased long-term survival, whereas lower lymphoid:fibrohistiocytic proportions and higher mitotic index indicated a probability of higher short-term mortality.