Canine cystic astrocytomas: 7 cases.

Tumoral macrocysts (grossly observable cysts) are common in human pilocytic astrocytomas but are rarely reported in canine astrocytomas. Here we describe 7 canine astrocytomas with macrocysts. The median age of affected patients was 9.6 y, and 2 dogs were brachycephalic. Clinical signs reported in 6 cases included head tilt (4 cases), falling (4 cases), and ataxia (3 cases). Gross changes consisted of well (5 cases) or poorly demarcated (2 cases), tan-to-pale-yellow masses with distinct, 2-10-mm fluid-filled cysts that expanded the cerebellum (4 cases) or telencephalon (3 cases). Histology and immunohistochemistry revealed low-grade astrocytoma in 6 cases and high-grade astrocytoma in 1 case. Neoplastic cells were elongate, formed bundles embedded in an eosinophilic fibrillary stroma, and had oval-to-elongate nuclei. Gemistocytic differentiation was common (4 cases). Cystic areas were lined by neoplastic astrocytes and were either apparently empty or contained scant, eosinophilic, proteinaceous material and a few neoplastic astrocytes, foamy macrophages, and occasionally other leukocytes. The mitotic activity was absent or low across cases. Cytoplasmic immunolabeling for glial fibrillary acidic protein was detected in 6 cases. Our findings indicate that canine cystic astrocytomas are rare (our cases span several decades) and are located mainly, but not exclusively, in the cerebellum.