@Gary,
The NCCN Guidelines suggest PET-CT be used in almost all OC situations.

Per the NCCN Guidelines, a standard CT or MRI may be acceptable, but in many case only if a PET-CT is not available.

The Guidelines also recommend PET-CT be use for the initial follow-up after chemo/RT (UNLESS failure is suspected and a scan is need within eight weeks of ending RT).

Please re-review the "NCCN Practice Guidelines in Oncology V.2.2010 - Head and Neck Cancers".

If you download a PDF copy, just search it for "PET-CT".

We need to keep in mind that PET-CT technology is dynamic. I am not aware of any CCC'S in my area that use a PET only scanner (obsolete technology).

It is my understanding that a "state-of-the-art" CT and MRI are complimentary studies and, in most cases, a PET-CT (or more recently a PET-MRI) will adequately overcome the weaknesses of either alone.