Abstract
Background:
Cutaneous metastases of tumors are rare in dogs. In particular, only a few cases of cutaneous metastases from mammary gland tumors have been reported.

Case Description:
A 13-year-old neutered female Poodle presented with localized pruritus of the right axilla. Physical examination revealed redness and crusting. A skin taping test confirmed Malassezia infection. However, despite antifungal treatment, erythema and pruritus worsened, accompanied by increased crust formation. Immunosuppressive therapy was briefly attempted for a suspected immune-mediated disorder; however, it was ineffective. Subsequent skin biopsy and histopathological evaluation revealed adenocarcinoma with widespread vascular invasion. The tumor was positive for cytokeratin 7 and negative for cytokeratin 20, estrogen receptor, progesterone receptor, and p63. Multiple mammary masses were also palpated. Based on the clinical and histological findings, a diagnosis of metastatic mammary adenocarcinoma was made. Although palliative therapy was initiated, the dog developed acute respiratory distress 2 weeks after the biopsy. Thoracic radiographs showed multiple pulmonary nodules, which were presumed to be metastases.

Conclusion:
Although metastatic skin tumors are rare in dogs, they can mimic dermatitis and should be included in the differential diagnosis.

Key words: Dog; Malignant; Mammary gland tumor; Metastasis; Skin.