Canine cutaneous clear cell adnexal carcinoma (CCCCAC) is a rare primary cutaneous adnexal neoplasm. Microscopically, CCCCAC is characterized by dermal and/or subcutaneous lobules of pleomorphic polygonal to spindle-shaped cells with clear, glycogen-rich cytoplasm containing variably sized vacuoles with indistinct borders frequently associated with peripheral palisading, follicular papillary mesenchymal body–like structures, comedonecrosis, and lymphovascular invasion. Despite aggressive microscopic features, the biological behavior of CCCCAC in dogs remains poorly defined, and metastatic rates are likely underestimated. This report describes a metastatic case of CCCCAC in an 11-year-old mixed-breed, neutered male dog presenting with a cervical cutaneous mass and progressive pulmonary and lymph node metastases. Fine-needle aspiration cytology supported the diagnosis of a malignant epithelial neoplasm. Histopathology revealed a multilobulated, poorly demarcated, infiltrative neoplasm composed of clear cells with marked pleomorphism, high mitotic activity, comedonecrosis, and extensive lymphovascular invasion. Histochemical staining demonstrated cytoplasmic glycogen accumulation by periodic acid–Schiff positivity with diastase sensitivity. Immunohistochemically, neoplastic cells showed diffuse cytoplasmic immunoreactivity for pan-cytokeratin, variable expression of vimentin and S-100, and absence of melan-A, smooth muscle actin, and thyroid transcription factor-1 labeling, confirming the diagnosis of CCCCAC. Disease progression occurred despite supportive care, and necropsy revealed marked local aggressiveness and widespread metastases to lymph nodes, lungs, parietal pleura, and pancreas. To the authors’ knowledge, this is the first documented case of metastatic CCCCAC in Brazil.
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