| Summary: | Tumors of sweat glands usually originate from apocrine glands and can develop throughout the body but are rare in dogs. This report describes the retroperitoneal metastasis of primary cutaneous apocrine adenocarcinoma. An 8-year-old, spayed female beagle dog, weighing 11.7 kg, presented with a history of anorexia, hypodynamia, and weight loss. Clinical examination, radiography, ultrasonography, and computed tomography revealed a skin mass on the dorsum of the right metatarsal region, an enlarged ipsilateral popliteal lymph node, and a retroperitoneal mass. Fine-needle aspiration cytology of the popliteal lymph node suggested metastasis of an apocrine sweat gland tumor. Surgical excision of the skin mass, popliteal lymph node, and retroperitoneal mass was performed. The retroperitoneal mass was diagnosed as a metastasis of primary cutaneous apocrine adenocarcinoma. Immunohistochemistry revealed that the tumor cells were positive for cytokeratin 7 but negative for cytokeratin 20 and S100 proteins. There were no postoperative complications, except for temporary hindlimb edema, including local recurrence or metastasis, in the 6-month postoperative follow-up period. This case illustrates that although malignant apocrine gland tumors are rare in dogs, a wide resection of primary cutaneous apocrine gland adenocarcinomas is recommended because of the risk of local invasion or distant metastasis.
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