Image Challenge This 63-year-old man with a 10-year history of progressive neck enlargement noted a sudden increase in the size of the swelling on the left side.

This 63-year-old man with a 10-year history of progressive neck enlargement noted a sudden increase in the size of the swelling on the left side.

Question

This 63-year-old man with a 10-year history of progressive neck enlargement noted a sudden increase in the size of the swelling on the left side. What is the most likely diagnosis?