Metanephrine-secreting adrenocortical carcinoma in a dog with clinical and biochemical features suggestive of pheochromocytoma.

Adrenocortical carcinoma can secrete metanephrines and mimic pheochromocytoma; immunohistochemistry is essential for accurate diagnosis.

This case report describes the first confirmed metanephrine-secreting adrenocortical carcinoma (ACC) in a dog, presenting with clinical and biochemical findings mimicking pheochromocytoma. A 12-year-old castrated male Maltese was evaluated for an incidentally detected left adrenal mass accompanied by persistent systemic hypertension (160-210 mmHg) and elevated urinary normetanephrine and metanephrine-to-creatinine ratios. Computed tomography revealed a homogeneously enhancing adrenal mass without evidence of vascular invasion. Following preoperative alpha-adrenergic blockade with phenoxybenzamine, laparoscopic adrenalectomy was successfully performed. Histopathological examination confirmed ACC with vascular tumor emboli. Immunohistochemical profiling demonstrated diffuse Melan-A positivity and chromogranin A negativity, with focal synaptophysin immunoreactivity on serial sections, indicating adrenocortical origin with partial neuroendocrine differentiation—a pattern consistent with rare catecholamine-secreting ACC reported in humans. Postoperatively, urinary catecholamine metabolite concentrations normalized, and blood pressure remained within normal limits without antihypertensive therapy throughout an 11-month follow-up period, confirming the ACC as the source of the biochemical and hemodynamic abnormalities. This case highlights that elevated urinary metanephrines and systemic hypertension in dogs do not exclusively indicate pheochromocytoma; ACC should be considered in the differential diagnosis. Accurate diagnosis requires comprehensive immunohistochemical evaluation, as treatment protocols and prognostic implications differ significantly between ACC and pheochromocytoma. Clinicians should maintain a broad differential when evaluating adrenal masses with catecholamine excess.

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