The spread of cancer, seeding, via a FNAB is rare. In rarity, it's occurrence were with larger core needle biopsy in the breast, and have read some in the thyroid, but it's widely used, and considered safe, and an alternative to a surgical biopsy, which has its risk too. Some say, doctors that is, that a PET/CT is 90% accurate at 6-8 weeks, earlier has more false positives, 3 months or 12 weeks is when most have a scan is more accurate, 4 months is more accurate, and at 12 months, it's 100%. The is no general consensus when any period scanning should be done post treatment, in absence of any suspicion, and is only recommended for certain T3, T4 disease only, per NCCN guidelines in the oropharynx, larynx, etc. none in oral cancer, but each doctor, hospital may have different procedures.