A 70 years old hypertensive male came to the surgery opd with a history of persistent dysphagia and a round obliterated swelling since last 10 days in neck. There is no exophthalmos or loss of weight present. The built is moderate and no loss of appetite or difficulty in sleep, bowel or bladder movement noticed. On examination, the swelling was round, on right side of neck, firm, well defined margins were present and no complain of pain was noticed. The trachea was however deviated to the left side. What is the probable diagnosis??