Actually, there is no general consensus for any post treatment scanning surveillance, and according to NCCN Guidelines, only certain cancers like the oropharynx, hypo-pharynx, larynx, T3-4 disease only, and not oral cancer, say to have one 3-6 months post treatment, and that's as far as it goes in absence of any signs of cancer or recurrence, so it depends on the doctor, hospital and even the patient themselves on what type of scan, if anymore, can be done.

All my doctors I had did and do a post treatment scan from 3 months, sometimes was 4, 5, even later. Mostly they all were PET/CT, but the follow-up scan, 6 months later, may have been an MRI since I can't have the CT contrast. Even with an MRI they want your GFR around 35, and just made that. Some doctors like to switch them out PT/CT then a CT. My first time with cancer, and with induction chemo in 2009, I was so sick, weak, I couldn't have a PET/CT until 9 months later.

There are some that say a thorough visual and palpable examination is just as good, even a self exam, which is true to some extent, and the fact that I found all my cancers, 6, myself except for one, but studies show that a PET/CT is superior, and finds cancer more often, which can go as low as 5mm. Now with a pec flap, 5 neck dissections, a palpable exam is almost useless, so I require surveillance scanning with my history of recurrences, and have had about 26 of them.

Also, with HPV oropharynx cancer, the rate of recurrence seems to be increasing in some 3 years later, even 5, in distant areas, some not normally associated with distant metastases, while non HPV levels off after 2 years, so they are saying not to ignore unusual complaints and consider while body scanning like PET/CT.

I hope this helps