A 56-year-old woman has noticed an enlarging lump on the right side of her neck for the past 7 months. On physical examination, there is a 3-cm nodule in the right upper neck, medial to the sternocleidomastoid muscle and lateral to the trachea at the angle of the mandible. CT scan shows a circumscribed, solid mass adjacent to the carotid bifurcation. Microscopic examination of the excised mass shows nests of round cells with pink, granular cytoplasm. Tests for immunohistochemical markers chromogranin and S-100 are positive. Electron microscopy shows neurosecretory granules in the tumor cell cytoplasm. The tumor recurs 1 year later and is again excised. What is the most likely diagnosis?

Paragangliomas are neuroendocrine tumors that rarely produce sufficient catecholamines to affect blood pressure, in contrast to their adrenal medullary counterpart, pheochromocytoma. The microscopic appearance of these lesions does not always correlate with their biological behavior. There is a tendency for recurrence and metastasis despite the tumor's "bland" appearance. Metastases always should be considered in patients this age. About 5% of squamous cell carcinomas of the head and neck manifest initially as a nodal metastasis, without an obvious primary site, but the microscopic pattern here is not that of squamous cell carcinoma. Some thyroid cancers initially may manifest as a nodal metastasis, but the microscopic pattern in this case fits best with paraganglioma. A mucoepidermoid carcinoma or a Warthin tumor arises in a salivary gland.