To add to the discussion, we're currently at MD Anderson & yesterday we too asked the ocologist for a PET/CT fusion test at this visit. (My husband,57, is 14 months out from treatment for SCC base of tongue,Stage 2/3, HPV 16? positive tumor'non-smoker,non-drinker.)The answer was they use the fusion when they suspect metastisis. The CT is 'more fefined' & the radiologists prefer the CT. The MRI is not used for the 'lower' areas, i.e. base of tongue. In addition to the CT, at each 6 month visit, my husband is given an ultrasound of the lymph nodes in the neck & if any are suspicious, he would have a needle biopsy. We cannot get the fusion test currently in Ontario, nor do they ultrasound his lymph nodes,that's why we come to this wonderful place.