Clinical, diagnostic, and therapeutic characteristics of low-grade mammary carcinoma and malignant myoepithelioma in a female dog

Mammary neoplasms represent one of the most common tumor conditions in female dogs, particularly affecting older and unspayed females. Within this context, carcinoma and malignant myoepithelioma, considered rare in this species, exhibit aggressive clinical behavior even in low histological grade cases, consisting of both myoepithelial and epithelial components. Given its uncommon occurrence and the scarcity of reports, this case report aimed to highlight the clinical, diagnostic, macroscopic and microscopic features, therapeutic approaches, and oncologic follow-up associated with grade I carcinoma and malignant myoepithelioma, with the goal of disseminating knowledge about its biological behavior within the scientific community. A 13-year-old mixed-breed female dog presented at the Veterinary Hospital of the University of Franca (UNIFRAN) with pseudocyesis and a mammary nodule in the right abdominal chain. After clinical evaluation, complementary tests, and total right mastectomy combined with ovariosalpingohysterectomy, histopathological examination confirmed the diagnosis of grade I carcinoma and malignant myoepithelioma, with metastasis to the inguinal lymph node. One year after surgery, the patient developed contralateral recurrence and pulmonary metastases. Palliative treatment with oral chemotherapy and supportive therapy was initiated, resulting in neoplastic stability and maintained quality of life after six months of follow-up. This case highlights that, although rare, carcinoma and malignant myoepithelioma should be included in the differential diagnosis of neoplasms with both epithelial and myoepithelial features. It also emphasizes the importance of early diagnosis, multimodal therapeutic approaches, and systematic oncologic follow-up to ensure quality of life and greater longevity in affected patients.