Carcinoma of unknown primary site: a clinical enigma.

Management of carcinoma of unknown primary site (CUPS) is determined by tumor histologic characteristics and clinical presentation. Undifferentiated carcinoma or adenocarcinomas comprise the majority of these tumors with typical occult primary sites in the pancreas, lung, liver, or gastrointestinal tract. Elaborate diagnostic evaluations are of little benefit and lack of impact on treatment. Squamous carcinomas usually represent lymphatic metastases of head and neck tumors, which may be controlled for extended periods by irradiation. Unusual presentations of CUPS in which the histologic condition is adenocarcinoma or undifferentiated carcinoma are important to recognize because of the potential for specific therapy or cure of such entities as breast cancer and germ cell tumors.