Occult primary head and neck carcinoma: Role of positron emission tomography imaging

The PET study can identify the occult primary tumor in ∼30% of patients who present with an unknown primary tumor and have no detectable tumor using conventional methods including fiber-optic endoscopy and CT/MRI imaging. New generation PET scanners and PET-CT fusion may increase the accuracy and effectiveness in head and neck cancer patients. The fused PET-CT is superior to either modality alone in the assessment of head and neck malignancies. The ability to identify the occult primary tumor is imperative because it allows site-specific therapy in a more directed fashion and avoids the side effects of wide field radiation. The principle of PET imaging is based on the coincident detection of two opposite emitted gamma photons from the annihilation reaction between a positron and an electron. The most commonly used and clinically useful PET radiopharmaceutical is 18F-2-fluoro-2-deoxyglucose (18F-FDG), a glucose analog that is preferentially taken up by various types of neoplastic cells. 18F-FDG-PET imaging is valuable in head and neck cancer diagnosis, cancer staging, and assessing response to therapy. The future of PET imaging in head and neck cancer is likely to mature with the development of new PET tracers that allow for more accurate tumor detection with less uptake by the normal tissue. The new PET tracer carbon-11-choline detects squamous cell carcinoma with the accuracy equivalent to FDG-PET but has the advantage of less muscle uptake and shorter examination times.