Firm, Nontender Mass on the Neck: Your Dx?

For several years, an asymptomatic, firm, nontender mass had been growing on the neck of a 54-year-old-man, slightly left of midline at the level of the hyoid bone. The mass was mobile on protrusion of the tongue and during swallowing. Thyroid function tests were normal. An axial thoracic CT scan was obtained.