Are you talking about follow up scans?
I got PET scans, every 6 months for first year after treatment ended. Yearly for 4 years after that. one more after 6 years. My oncologist told me that unlike breast cancer, head neck cancers, if they come back, usually (99% of the time) appear within 2 years. So six clean years made me good to go. smile
BTW I was Squamous cell / tonsil source with lymph node involvment, Stage 4,

Stage 4 squamous cell tonsil cancer. Two lymph nodes involved surgically removed before source found to be tonsil. Implant radiation right on tonsil, regular mask radiation therapy for two months and a final Implant radiation. All clear confirmed by PET scans for 6 years.