Dear Harish Hello I am Dr Ravinder Sharma ENT surgeon. On the basisi of the info you have provided I will post my opinion --- I understand your concern about something serious as a cause of your neck swelling. I would advise to undergo a test FNAC - fine needle aspiration cytology for the neck swelling and a panendoscopy --- it comprises of nasal endoscopy, nasopharyngoscopy, laryngoscopy
and oesophagoscopy. Even if there is slightest doubt all these investigations can actually tell you about the disease you are suffering from.
Its not that you are a hypochondriac. Your concerns are genuine and can be disturbing to anyone who is in your situation. A relatively non invasive test as compared to panendoscopy can be PET CT scan
to look for the primary. If you have a cancer you get to know the extent of lesion and if it is not there you get a clear NO.
All these tests have some limittions and may miss very small lesions. But then in real world this is the best way to get an answer. You can increase the sensitivity of these tests by regular follow up and repeat test -- say every 3 months
I hope this answers your concerns Do let me know if I can help you futher about it. with regards