Abstract
Background:
Colloid cysts are benign intracranial lesions of endodermal origin, typically arising in the third ventricle of the human brain. Although uncommon, they may cause clinical signs when obstructing cerebrospinal fluid pathways. While cases in atypical sites such as the cerebellum or pontomesencephalic region have occasionally been described, no reports exist of colloid cysts within the cerebellopontine angle (CPA) in either veterinary or human medicine.

Case Description:
A 5-year-old castrated male Pomeranian presented with ataxia and excessive salivation. Neurologic examination was largely unremarkable apart from gait disturbance. Magnetic resonance imaging revealed a well-defined cystic mass in the CPA measuring 12.7 × 12.7 × 10.8 mm, exerting compressive effects on the cerebellum and medulla. The lesion displayed gravity-dependent signal layering and peripheral rim enhancement, suggesting a benign encapsulated cyst. Based on location and imaging features, differentials included epidermoid or dermoid cysts. Surgical removal via dorsal craniectomy and durotomy was performed to alleviate compression and establish a definitive diagnosis. Histopathology identified a fibrous-walled cyst lined by ciliated cuboidal to columnar epithelium with papillary projections and no evidence of keratin, adnexal elements, or neoplasia. The findings were consistent with a colloid cyst. The patient made a complete recovery and remained clinically asymptomatic and neurologically intact during 10-month follow-up.

Conclusion:
This case represents the first known instance of a colloid cyst occurring in the CPA of a dog. Imaging and histologic features closely mirrored those described in human cases. The report highlights the need to consider colloid cysts among differential diagnoses for CPA masses in dogs and underscores the value of histopathological confirmation in cystic brain lesions presenting in atypical locations.

Key words: Colloid cyst; Cerebellopontine angle; Dog; Intracranial cystic lesion; Surgical management.