@Cheryl
I came across this link to the BC Cancer Agency guidelines for Head & Neck Cancer:

http://www.bccancer.bc.ca/HPI/CancerManagementGuidelines/HeadnNeck/default.htm

I would think this would be the same throughout Canada.

The Diagnostics section goes into the pros and cons of CT's, MRI's and PET scans along with recommendations on when each type of scan should be used.

These guidelines, similar to our NCCN guidelines, seem to set a minimum level of care.