CONTROL ID: 2016971 TITLE: Use of Iba1 Immunohistochemistry in the Diagnosis of Hepatic Histiocytic Sarcoma in a Dog: A Case for Kupffer Cell Neoplasia? ABSTRACT BODY: Narrative: Common sites of origin of histiocytic sarcoma complex in dogs

BODY: Narrative: Common sites of origin of histiocytic sarcoma complex in dogs include the skin, subcutis, joints, lymph node, spleen, lung, and bone marrow. Iba1 expression has been reported in subdural (Ide 2011) and ocular histiocytic sarcoma but not in other sites. A 7-yearold male neutered border collie presented with chronic weight loss and a 2 day history of vomiting and anorexia. CBC and chemistry demonstrated an albumin of 1.7 (2.6-4.2) and mild signs of cholestasis. Coagulation times were mildly elevated. Following diagnostic imaging and a short hospitalization the dog developed severe hemoabdomen and was euthanized. At postmortem examination approximately 90% of the left lateral liver lobe was expanded and effaced by coalescing, tan to yellow, firm, irregularly defined nodules. Similar nodules, ranging in diameter from 1 to 3cm, were found throughout all lobes. Additional findings included multifocal dark red nodules up to 2.5cm in diameter expanding the spleen, and enlargement of hepatic lymph nodes up to 3cm in diameter. Cytology of the hepatic nodules demonstrated pyogranulomatous inflammation with atypical histiocytes. Histopathologically, neoplastic cells filled hepatic sinusoids and formed unencapsulated, poorly demarcated, infiltrative, moderately cellular nodules composed of ill-defined packets. Cell morphology varied from spindle to polygonal to round with indistinct borders and moderate to abundant, pale, eosinophilic, often foamy cytoplasm. Nuclei were pleomorphic and ranged from spindleshaped with clumped to marginated chromatin to round/oval nuclei with coarse chromatin and one to two distinct nucleoli. Mitoses averaged 20 per 10 HPF and were occasionally bizarre. Scattered neoplastic cells were multinucleate and no erythrophagocytosis was seen. Similar neoplastic cells effaced the hepatic lymph nodes and were seen in only 2 of the splenic nodules. Micrometastases were seen in the lung and tracheobronchial lymph node. Morphologically the neoplasm was preliminarily diagnosed as a poorly differentiated sarcoma. Histiocytic sarcoma (HS) was diagnosed based on neoplastic cells’ cytoplasmic expression of Iba1. Results were supported with follow-up immunohistochemistry (IHC) using CD3, CD18, and CD20. Experimental IHC was attempted using 2 clones of anti-mouse CLEC4F antibody but staining was nonspecific. Investigation to rule in or rule out a dendritic cell origin is ongoing. The findings in this case illustrate the value of Iba1 in the diagnosis of HS in the canine liver. The large hepatic tumor burden, combined with the absence of a clear extrahepatic primary neoplasm, raise the possibility of primary hepatic HS. CURRENT CATEGORY/DISCIPLINE: Pathology AUTHORS/INSTITUTIONS: K. Kim, R. Bauer, Louisiana Animal Disease Diagnostic Laboratory, Vanderbilt University, Baton Rouge, Louisiana, UNITED STATES; K. Kim, R. Bauer, Department of Pathobiological Sciences, Louisiana State University, Baton Rouge, Louisiana, UNITED STATES; AWARDS: Graduate Student Poster Presentation|Graduate Student Oral Presentation|AAVLD/ACVP Pathology Award Trainee Letter: K Kim Role and Significance.pdf