Hemangiosarcoma of the Third Eyelid in an American Pit Bull Terrier

Background: Hemangiosarcoma (HSA) is a malignant neoplasm arising from the endothelial cells of blood vessels. It has fast growth, and severe local infiltration and metastasis power, in addition to risk of hemorrhage due to the fragility of its vessels. HSA develops in dogs aged 8 to 13 years but can affect younger animals too. Ocular involvement in HSA is rare, but when identified, the third eyelid and bulbar conjunctiva close to the limbus are the most affected sites by this neoplasm. This study aimed to report the clinicopathological aspects of a case of HSA in the third eyelid of an American Pit Bull Terrier breed. Case: A 10-year-old male American Pit Bull Terrier with a history of a red hemorrhagic mass on the third eyelid of the left eye was examined at a veterinary clinic. On physical examination, the animal showed signs of ocular discomfort and bleeding. On ophthalmologic examination, a raised red mass, approximately 2 cm in diameter, was identified on the anterior surface of the third eyelid. The mass was surgically excised. The excised tissue fragment was fixed in 10% buffered formalin solution for 24 h and sent for histopathological examination. Macroscopically, the fragment was irregular, soft, and brownish and measured 2.0 × 1.0 × 0.5 cm. Histologically, proliferation of non-delimited and non-encapsulated atypical endothelial cells, which were organized in vascular arrangements forming small lakes filled with red blood cells, was observed. The cells exhibited elongated and basophilic cytoplasm, oval nuclei with coarse chromatin, and evident nucleoli. Moderate anisocytosis and anisocariosis were observed, with no mitotic figures. Epithelial hyperplasia with mild mixed inflammatory infiltrate was noted. Surgical margins were compromised. Sections of neoplastic tissue were processed for immunohistochemical evaluation with anti-CD31, anti-factor VIII, and anti-Ki-67 antibodies. Neoplastic cells exhibited marked immunostaining for CD31 and factor VIII, and only 8% of these cells were immunostained for Ki-67. Discussion: The diagnosis of HSA in the third eyelid was based on histological features and positive immunostaining for CD31 and factor VIII. The Ki 67 protein is a marker of cell proliferation, being highly expressed in malignant cells, and has been applied as a prognostic marker for different types of neoplasms. Hemangiosarcoma of the third eyelid is a rare malignant neoplasm in small animals. Dogs are the species most affected by this tumor, with the incidence age varying from 8 to 13 years; however, it can also affect younger animals. Animals with thin, light hair and glabrous regions, especially on the face and periocular region, may be more predisposed to this neoplasm. Surgical excision with a wide margin of safety is the recommended treatment for HSA. In addition, chemotherapy may be indicated as a complement to the surgical procedure, especially if the margins are compromised. The main chemotherapy protocols used for this neoplasm include VAC I and VAC II, which are associated with the drugs, doxorubicin, vincristine, and cyclophosphamide. Another alternative to conventional protocols is the use of metronomic chemotherapy, which involves intensifying an anti-tumor immune response and decreasing tumor vascular density. Differential diagnoses for hemangiosarcoma (HSA) of the third eyelid in dogs include other neoplasms with ocular-conjunctival involvement, such as third eyelid gland adenocarcinoma, conjunctival melanoma, mastocytoma, and squamous cell carcinoma. Keywords: angiosarcoma, HSA, malignant neoplasm, immunohistochemistry, eye, ocular, dog.