Tympanic papillary adenoma (tympanic papilloma) in a cat - a case report.

A 9-year-old cat presented with an inflammatory discharge and a polypoid mass located in the left ear canal, penetrating the tympanic membrane. The polypoid mass was removed surgically using traction-avulsion but not submitted for histological examination. Subsequently, the cat underwent computed tomography of the head, which revealed an irregular mass located in the left tympanic bulla. The mass was resected surgically during external ear canal ablation, with lateral osteotomy of the tympanic bulla, and submitted for histological examination. The tumour consisted of papillary structures lined by a single layer of cylindrical, ciliated cells with basally oriented, oval nuclei, minimal atypia, very low mitotic count and periodic acid-Schiff-positive mucin within the cytoplasm. The tumour cells immunolabelled for cytokeratins, and the Ki67 index was 10.4. Based on these findings, tympanic papillary adenoma was diagnosed. Although the clinical signs initially resolved after surgery, the tumour recurred within 17 months, as confirmed by histological examination. The recurred tumour showed an increased mitotic count and Ki67 index (18.6) compared with the primary tumour. This case report presents the clinical outcome, radiological and histological features of tympanic papillary adenoma in a cat, a very rare entity with no direct analogue in human pathology.