A case of suspected ectopic parathyroid chief cell carcinoma in a cat.

A 12-year-old spayed female Korean shorthair cat was evaluated for mediastinal and cervical masses. A serum biochemistry analysis revealed hypercalcemia and hypophosphatemia, with normal parathyroid hormone (PTH) levels. Diagnostic imaging showed multicystic cervical and mediastinal masses with increased opacity in the left caudal lung field, suggestive of partial lung collapse. Abdominal imaging was unremarkable. Cytology of the mediastinal mass suggested an epithelial or neuroendocrine tumor. Hypercalcemia remained unresolved following bisphosphonate therapy; the cat was euthanized 290 days after presentation. Postmortem examination revealed multicystic cervical and mediastinal masses with hemorrhage and necrosis. Histopathology showed neoplastic epithelial cells with cystic degeneration and lymphatic invasion in cervical and mediastinal masses. Immunohistochemistry confirmed PTH expression, establishing the chief cell parathyroid carcinoma. To our knowledge, this case represents a suspected ectopic feline parathyroid chief cell carcinoma with extensive cystic change and metastatic features.