The clinical presentation and histopathologic-immunohistochemical classification of histiocytic sarcomas in the Flat Coated Retriever

The Flat Coated Retriever is a breed at risk of development of histiocytic sarcoma (HS), but in contrast to the disseminated form of disease recognized in the Bernese Mountain Dog, most reports of HS in Flat Coated Retrievers describe a localized lesion affecting the musculature or fascia of limbs. The purpose of this study was to review data and material received though an ongoing Flat Coated Retriever tumor survey to better define the presentation of HS in the breed and to determine the utility of subclassification of tumors arising at different sites by histology and immunohistologic phenotyping. Data on 180 dogs bearing HS-like tumors were available for review, which showed that although the majority (101 lesions, 57%) were primary limb lesions, 47 dogs (26%) had visceral, mainly splenic lesions with no peripheral primary tumor. A detailed histologic and immunohistologic review of 20 limb tumors and 20 splenic tumors showed that 2 distinct phenotypic subtypes could be identified: a histiocytic subtype, most prevalent in the splenic tumors, and a histiocytic-spindle-pleomorphic subtype, mainly seen in the limb tumors. Despite their variable morphology, all tumors expressed major histocompatibility complex class II and the leukocyte antigen CD18, but only those tumors in the spleen consistently expressed CD11d. The majority of tumors also contained a mild to moderate infiltrate of T lymphocytes.