Cancers of unknown primary origin (CUP) represent a heterogeneous group of tumors for which the primary site lesion remains unidentified after through medical history, clinical examination and extensive diagnostic workup.
The purpose of our study was to contribute in the value of 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (18F-FDG) positron emission tomography/computed tomography (PET/TC) in the detection of CUP in patients with histologically proven tumor metastasis and/or a high clinical suspicion of metastatic diffusion.
The study population consisted of 172 consecutive patients unsuccessfully studied, within the previous two months, with physical examination, laboratory investigations, conventional diagnostic procedures (including RX/Ecography/CT/MRI/endoscopy). All patients were studied by 18F-FDG PET performed according to the standard procedure (6 h of fasting, intravenous injection of 370 Mbq of 18F-FDG and image acquisition with a PET/CT scanner for 4 min per bed position). The images obtained were analyzed with visual and semi-quantitative methods; histopathology, cytology and/or clinic/diagnostical follow up were used to evaluate the PET/CT results.
The primary tumor site was correctly identified by 18F-FDG-PET/TC in 56/172 cases (32.5 %): lung (n=11), rino/oro-pharinx (n=9), lymphatic system (n=6), colon (n=6), liver (n=4), ovary (n=4), pancreas (n=3), mammary gland (n=3), muscles-endothelium (n=2), stomach (n=2), testicle (n=1), mesothelium (n=1), prostate (n=1), thyroid (n=1), parotid (n=1), kidney (n=1). FDG PET/CT findings were false positive in 17 of 172 patients (9,8%), identifying the primary cancer in the lungs (n=10), the head and neck region (n=6) and the ileocolonic area (n=1). In 77 subjects 18 F-FDG-PET/TC scans were negative (77/172; 44.7%) and in 12 of them histopathology revealed a primary localization (false negative 15.5%). In 95 patients (95/172, 55.2%) the primary tumor was not identified after extensive conventional diagnostic workup and 18 F-FDG-PET/CT scan, even though in 21 of them (21/95 22.1%) our methodology detected further unexpected metastases. Furthermore 18 F-FDG PET/CT detected new metastatic sites in 48 patients of the whole population (48/172, 27.9%), modifying the stage of the disease and the subsequent clinical management.
Our data support the valuable diagnostic role of 18F-FDG-PET/TC in the evaluation of patients with CUP, depicting correctly the primary tumor site in 32.5% of patients and having an important contribution in the detection of unexpected metastatic sites modifying the stage of the disease in 27.9% of patients.