Hello Barry @grimmers and welcome to head and neck connect. My radiation oncologist 13 years ago said that I would have some memory loss from the post surgery radiation to my skull base by my right ear. He was older and quite experienced at University of Michigan and recommended by Mayo radiation oncologists for treatment closer to home. Now at age 71 my memory symptoms and fogginess might also be from age. There is no sudden change, but very gradual with some days better than others. I would suggest a referral to a neurologist who specializes in dementia as they are the ones to do testing and give a diagnosis. Since you already have the brain MRI that will be helpful to a neurologist.
I just read a good book which might interest you called Heal Your Gut, Save Your Brain by Partha Nandi MD. There is a lot of explanation of the link between GI and brain health.
Do you thing that your PCP or oncologist will make that referral to ease your mind abut your symptoms?