ABSTRACT
Background Cancer of unknown primary (CUP) is defined as a biopsy-proven secondary lesion with no detectable primary tumor after physical examination and conventional imaging tests (X-ray imaging, computed tomography (CT), and ultrasound (US)). The primary site of some cancers may eventually be found by additional tests, and in this case they are no longer considered CUP. The reasons why the primary tumor sometimes is not found can be different: (1) sometimes the secondaries have grown very quickly while the primary remains very small or below the resolution of the diagnostic modalities; (2) some immunitary reactions can cause the spontaneous disappearance of the primary tumor; and (3) the diagnostic imaging methods are not able to depict the primary because it is hidden.1 The frequency of CUP in cancer varies from 2-3 to 4.2%, depending on geographical area, and represents the eighth most frequently occurring malignancy in the world. CUP prognosis is poor: fewer than 25% and 10% of patients are alive at 1 and 5 years respectively.2,3
The histological types have been classified according to the World Health Organization (WHO) into: adenocarcinoma (well to moderately differentiated and poorly or undifferentiated), squamous cell carcinoma (undifferentiated and not specified), neuroendocrine tumors, lymphomas, germ cell tumors, melanomas, sarcomas, and embryonal malignancies.4 The diagnosis of CUP can be made on the basis of symptoms, which depend on the organ the CUP has spread to.
