I've just read through the NCCN follow-up recommendations and it says

"Post treatment baseline imaging of primary (and neck if treated) recommended within 6 mo of treatment*. Further reimaging as indicated based on signs/symptoms; not routinely recommended for asymptomatic patients."

and also later clarifies "For cancer of the orapharynx ..... imaging recommended for T3-4 and N2-3 disease only."
Why do they recommend this course of (non)action. Is it just because of the concern with false positives or excessive radiation? If so, why don't they do fine-needle aspirations and biopsies of the treatment areas? Or do they assume all is oK there and the potential problem is more distant metastases?