Correctly suggested by your physician to get a CT done.

start nasal decongestion and jal neti for atleast 3 weeks

Thanks for your question. infact all of us have sinus rather doctor means to say you are having sinusitis- infection or inflammation of sinuses.But there are exact positive and negative history which can lead to diagnosis of sinusitis.Also you are correct that recurrent exposure to radiation can lead to harmful effects.We the ENT and Head neck Surgeons usually ask for CT scan if clinically indicated or before surgery or if any recurrence of disease post op suspected etc etc as per protocol.Headache is a broad entity.I would suggest you to consult an ENT specialist rather before doing any CT PNS as sometimes there are other tests like clinical exami,NE also which can diagnose the cause if indicated.