Head and neck cancer: carcinoma of unknown primary.

Carcinoma of unknown primary is uncommon, estimated to represent only 3% to 5% of all head and neck cancers. Squamous cell carcinoma accounts for 70% to 90% of these lesions, most commonly from sites in the upper aerodigestive tract, including tonsils, base of tongue, nasopharynx, and piriform sinus. Magnetic resonance, computed tomography (CT), and positron emission tomography all play a role in the assessment of patients with an unknown primary. The location of a metastatic lymph node may give an indication of the primary tumor site, and knowledge of lymph node drainage patterns is important for anyone evaluating these patients. Magnetic resonance and CT are both used for evaluation for extracapsular nodal disease, perineural tumor spread, osseous skull base or perivertebral space involvement, and vascular invasion, findings that seriously impact treatment and prognosis. Positron emission tomography/CT also plays a significant role in detecting primary tumor sites and identifying distant metastatic disease.