Metastatic disease in a dog with a well-differentiated perianal gland tumor

Fine-needle aspirates from a perianal mass on an 8-year-old, intact male, Miniature Poodle presenting for tenesmus showed a uniform population of well-differentiated hepatoid cells with no notable criteria of malignancy. The cytologic diagnosis was a perianal gland tumor, with adenoma likely given the cytomorphology. The abdominal ultrasound revealed multiple, markedly enlarged, intra-abdominal lymph nodes. LN aspirates also showed well-differentiated polygonal, hepatoid cells displaying no notable cellular atypia. The presence of the metastasis led to the interpretation of a well-differentiated, malignant perianal gland tumor despite the benign cellular appearance. Histopathology of the surgically excised perianal mass and one enlarged abdominal lymph node revealed lobules of uniform polygonal hepatoid cells arranged in organized islands and trabeculae surrounded by a single layer of uniform reserve cells. Few mitotic figures were present. The only histopathologic indication of malignancy within the primary mass was the presence of small islands of well-differentiated hepatoid cells infiltrating into adjacent tissue and possible lymphatic invasion. The histopathologic diagnosis was perianal gland adenocarcinoma. Most textbooks describe perianal gland adenocarcinomas as showing increased cellular atypia including pleomorphism, disorganization of hepatoid cells, and increased numbers of pleomorphic reserve cells with mitotic figures. This case is an example of the occurrence of a well-differentiated perianal gland tumor with metastasis and highlights the importance of realizing that with these tumors, a benign cytologic and histologic appearance may not correlate with biologic behavior. To the authors' knowledge, this is the first case reporting both the cytologic and histologic appearance of a well-differentiated metastatic hepatoid gland tumor.