Hi. Can anyone tell me how long after treatment the doctors usually wait before doing a PET/CT scan? Thank you.

Caretaker to my best friend, my mom. Age 60 - never smoked or drank. St IVA oropharynx. 37 radiation tx, 8 carboplatin tx. Diag 5/31/07-TX completed 8/16/07-good PET 10/10/07. Passed away 3/28/08 due to weakened blood vessels from tx. Now watching over her triplet grandsons born on 5/19/08.