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This represents the first published case report of mediastinal fibrosarcoma in a dog. An 8-year-old male neutered mixed breed dog was presented for evaluation of lethargy and increased panting. Thoracic focused assessment with sonography for trauma revealed moderate pleural effusion. Thoracic radiograph findings were suggestive of a cranial mediastinal mass. Computed tomography revealed a mass within the right ventral aspect of the cranial mediastinum. On surgical exploration, a cranial mediastinal mass with an adhesion to the right cranial lung lobe was identified and removed en-bloc using a vessel sealant device and requiring a partial lung lobectomy. Histopathology results described the cranial mediastinal mass as fibrosarcoma with reactive mesothelial cells identified within the sternal lymph node. The patient was treated with systemic chemotherapy following surgical removal. To date, the dog has survived 223 days following diagnosis with recurrence noted 161 days following diagnosis and radiation therapy was initiated. Primary cranial mediastinal fibrosarcoma while a seemingly rare cause of thoracic pathology in dogs, should be considered in the differential diagnosis for a cranial mediastinal mass., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 McGrath, Salyer, Seelmann, Lundberg, Leonard, Lorbach, Lumbrezer-Johnson, Hostnik, Tremolada, Lapsley and Selmic.)

Background: Angiosarcomas are a broad category of vascular origin neoplasms that are poorly characterized in veterinary species. Lymphangiosarcoma (LAS) is an uncommon type of angiosarcoma reported in humans and canines arising from lymphatic endothelium. LAS can be differentiated from other angiosarcomas in dogs based on expression of Prospero-related homeobox gene-1 (PROX-1) or lymphatic vessel endothelial receptor-1 (LYVE-1). Composite hemangioendothelioma (CHE) is a rare angiosarcoma subtype described in people and characterized by a variable biologic behavior and infrequent metastasis. This variant of angiosarcoma histologically combines features of retiform hemangioendothelioma and epithelioid hemangioendothelioma. Information regarding the cytologic and histopathologic appearance and clinical course of dogs with vascular tumors that exhibit features of CHE are unknown. Here, we report a case of pleomorphic LAS with features of CHE arising in a dog and treated with surgery and adjuvant chemotherapy. Case presentation: A 10-year-old intact male Labrador retriever presented with an approximately 6-cm-diameter cutaneous mass caudal to the left elbow that was progressively growing over 1.5 years. On physical examination, palpable extensions were identified coursing proximally over the triceps with concurrent loco-regional peripheral lymphadenopathy. Fine needle aspirates (FNA) and cytologic assessment of the cutaneous mass, left prescapular, and accessory axillary lymph nodes reported that this appeared to be a metastatic epithelial neoplasm, although a mixed carcinoma or collision tumor could not be excluded. An incisional biopsy of the mass was submitted for histopathology and was consistent with a well-differentiated angiosarcoma with features of CHE. The neoplasm expressed vimentin, CD31, von Willebrand factor (vWf), and PROX-1, supporting the diagnosis of LAS. Complete staging was performed, and no additional metastatic lesions were identified. Left forelimb amputation and lymph node removal were performed. Based on the diagnosis of metastatic LAS, doxorubicin chemotherapy was administered. 7 months post-amputation, the tumor recurred at the amputation site without evidence of metastatic disease. Conclusion: This report describes a malignant, locally aggressive lymphatic origin vascular tumor in a dog, with features consistent with descriptions of CHE in humans. Cytologic features in this case were discordant with its true mesenchymal etiology, obfuscating diagnosis. The morphologic features of the mesenchymal neoplastic population and immunohistochemistry (IHC) labeling ultimately supported a diagnosis of pleomorphic LAS with features of CHE., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Cook, Lorbach, White, Zann, Cianciolo, Selmic, Wavreille and Kisseberth.)