Histologic and Immunohistochemical Review of Splenic Fibrohistiocytic Nodules in Dogs

Background Splenic fibrohistiocytic nodules ( SFHN ) are commonly diagnosed. It is suspected that these represent a heterogeneous group of malignant and nonmalignant diseases, separation of which could improve the ability of clinicians to prognosticate for dogs. Hypothesis/Objectives Immunohistochemistry will differentiate histologic diagnoses within the group of SFHN ; survival after splenectomy is associated with those histologic types. Animals Thirty‐two dogs with SFHN treated by or under direction from veterinary oncologists. Methods Retrospective case record analysis from dogs followed from splenectomy until death. Clinical, histopathologic, and immunohistochemistry data analyzed for an association with survival time. Results Thirty‐two dogs had SFHN ; grade 1 (2 dogs), grade 2 (9 dogs), and grade 3 (lymphoid percentage <40%; 21 dogs). Twenty‐two dogs died, 10 were censored (9 alive median of 883 days after splenectomy). Median overall survival was 387 days, and grade 3 SFHN was positively associated with survival time as previously reported ( P  < .001). Of 31 available samples, dogs had diseases reclassified as nodular hyperplasia (13; 8 complex, 5 lymphoid including 2 marginal zone), lymphoma (4; 2 marginal zone lymphoma, 1 high grade B‐cell lymphoma, and 1 marginal zone transitional to high grade B‐cell lymphoma), 8 stromal sarcomas, and 6 histiocytic sarcomas. Dogs with histiocytic sarcoma had worse survival (median 74 days) than dogs with other diseases. Conclusions and Clinical Importance Splenic histiocytic sarcoma is an aggressive disease; however, some dogs with stromal sarcomas had long survival times. The term SFHN is no longer warranted for this group of disorders.