Tumors of unknown origin.

Tumors of unknown origin (TUOs) account for 3% of cancers and represent the eighth-most common cancer diagnosis. Although TUOs are commonly classified by their histological presentation, examination of the location of the metastases is equally important. When the histological type and location of metastases are both known, a finite number of common clinical syndromes emerge and one can predict the most common occult primaries.