The question is about the appropriate followup of head and neck cancer. This is surveillance rather than restaging. NCCN on p. 42, to which one is directed by links from unknown primary page, a PET is recommended at 12 weeks and thereafter imaging is optional. There are no prospective data demonstrating a survival benefit for any follow-up strategy in patients with treated head and neck cancer and available retrospective data are conflicting.
The detection of residual disease after therapy is a particularly difficult problem for patients who have head and neck cancer. Surgery, radiation therapy, and chemotherapy may result in anatomic changes that are difficult to differentiate from recurrent or residual tumor. CT and MRI frequently require serial studies to determine if tumor recurrence is present or absent. FDG-PET is not based on anatomic information, so the functional imaging of PET is able to provide an accurate determination of residual disease after therapy. Several studies have demonstrated the sensitivity of PET in the detection of recurrent or residual disease to be 81% to 100%, and the specificity to be 61% to 100%. The lower specificity results from the false-positive studies that occur with inflammatory lesions after radiation therapy, which may persist for up to 4 months after completion of therapy.
The most recent guideline is Scottish and it says: "Patients should have access to PET scanning, if appropriate, when recurrence is suspected."
Conessa C, Herve S, Foehrenbach H, Poncet JL.FDG-PET scan in local follow-up of irradiated head and neck squamous cell carcinomas. Otol Rhinol Laryngol. 2004 Aug;113(8):628-35.
William P. O'Meara, Follow-up of head and neck cancer patients post-radiotherapy Radiotherapy and Oncology, Volume 66, Issue 3, Page 323, 2002
Henze M, Rudat V, Dietz, et al. Monitoring of an organ preserving primary radiochemotherapy in carcinomas of the hypopharynx/larynx with 18F-FDG-PET. Program and abstracts of the 49th Annual Meeting of the Society of Nuclear Medicine; June 15-19, 2002; Los Angeles, California. Abstract 263.