[PET scan and double-independent pathologic investigations effectively support the detection of occult primary tumors].

Following the failure of conventional diagnostic procedures, whole-body FDG-PET investigations were carried out in 42 metastatic cancer patients to localize occult primary carcinomas. During the clinical follow-up, the presence of malignant tumor was ruled out in 3 cases, and 2 patients originally believed to have carcinoma were confirmed to be suffering from a malignant hematological disease. These false diagnoses were associated with the use of imaging methods only (2 cases) or cytology only (1 case), lack of double, independent pathological investigations (2 cases) or immunophenotyping (2 cases) and the occurrence of an unrecognized rare tumor in a hospital with a small patient turnover (1 case). The discovered 11 occult primaries (4 lung, 3 breast, 2 hypopharynx and 1 base of the tongue carcinomas and 1 non-Hodgkin lymphoma) led to a 28% diagnostic efficacy of PET (11/39 malignant pathological reports). This efficacy is doubled (60%) if PET reveals < or = 5 malignant lesions and the locations of the pathological foci are tumor-specific. We suggest PET investigations in the search for occult primaries following a controlled pathological diagnosis and the failure of conventional diagnostic procedures.