no later than 6 months post treatment for most, and see 3-6 months was changed for that, and even the post scan is recommended only for certain HNC, T3-4, N2-3 disease by the NCCN. After that, there are no set dates, and depends on the doctor, and hospital. My first post scan was 8 months due to medical reasons. I was never out of treatment after that for more than 6 months, so I had many scans, 12 or so in three years, but my doctors at two different CCC, mentioned every six months. My next scan is in August, and last one was in February.

I posted an article a few months ago about scanning on regular basis, after a post treatment scan, had little benefit, other than for any suspicions. Alternately, I did read elsewhere that PET's are more accurate in finding cancer, maybe in lymphs, more than a physical exam, which can go as small as 5mm, even before being felt.

You can always keep requesting one. Good luck