It may not feel like it, but you are doing quite well three months (12 weeks) after radiation. If you wish to put on weight, you can consider taking supplements like Boost or Ensure which are in liquid form and they can add to the calorie intake. (I don't know if you have Boost or Ensure in India. The supplements may have a different brand name.). Have you had a CT scan? Sometimes doctors wait a bit before they order a scan as the swelling post-radiation may mean that the scans are not that clear. Have you asked your doctor about that? Here in Toronto, Canada, doctors don't necessarily order a PET scan although it seems to be done quite frequently in the States.

Good for you that you are trying to be positive; that's really important in the recovery process.