Value of F-18 FDG PET/CT in diagnosis, potential prognosis and directing treatment of patients with metastases of unknown origin

Purpose: To assess the value of F-18 FDG PET/CT in metastases of unknown primary and its impact on management. Patients and methods: A prospective analysis of 123 patients, presenting to national cancer institute with pathology proven or clinico-radiologically suspected metastatic lesions of unknown primary. All patients had prior examination and routine workup including labs, radiologic investigations and histopathology before undergoing F-18 FDG PET/CT study, with a follow-up period of 12 months. The PET/CT was started from the level of the skull vault till the feet and we used high dose CT, the data was interpreted by both qualitative and quantitative manners and PERCIST criteria were used to evaluate response to treatment. Results: Study group included 66 males and 57 females with mean age 53.98±12.9 (range=23-85) years. Main sites of presenting metastases were bone (29%) and lymph nodes (22%) followed by liver (17%) and brain (11%). Histopathology revealed poorly differentiated carcinoma in 42%, however, 27% came with no pathology. PET/CT identified the primary in 71 patients with lungs (16 patients) followed by pancreas and breast (9 patients each) proving to be commonest origins of primary malignancy. It was false positive in 6 patients, and it excluded malignancy in 19 patients. False negative results were found in 27 studies, 24 of which remained unknown till the end of follow-up period. Sensitivity, specificity, PPV, NPV and accuracy were 72%,76%,79%,41% and 73% respectively. PET/CT identified additional site of metastases in 64 patients. Thus, it altered the management in 90 patients by avoiding unnecessary chemotherapy in negative patients or providing treatment targeting known primary tumor, including surgery with curative intent in 9% of patients causing better prognosis. During the follow-up period of the 71 patients, 12 patients had complete response, 12 patients showed partial response and 25 patients had stable disease. Conclusions: FDG PET/CT is a single modality that has several practical advantages for early detection of the primary tumor site in MUO patients facilitating early selection of appropriate treatment protocols that might improve patients' prognosis.