The value of F-18 FDG PET/CT for detecting primary foci in the metastatic cancer of unknown primary origin

Cancers of unknown primary origin (CUP) have poor prognosis and the median survival for patients with CUP is approximately 1 year. This survival can be extended by the identification of the primary origin and treating with specific therapy. F-18 FDG PET/CT scans of 75 patients (39 female, 36 male, mean age 60 ±12) with CUP referred to our clinic between January 2009 and January 2011 were evaluated retrospectively. Whole body images were obtained 60 minutes after the injection of approximately 370 MBq (10 mCi) F-18 FDG by PET/CT (Gemini-TOF-Philips). Emission scans were obtained for 1.5 min per bed position and transmission scans were obtained with low dose CT using 50 mA and 120 kvp. The tumor was identified histopathologically in 58 of 75 patients. 4 of 58 patients were treated as CUP. 17 of 75 patients could not be followed, so final diagnosis could not be made. In 54 patients, the primary was identified as 17 lung, 8 colorectal, 7 breast, 3 stomach, 3 pancreas, 2 endometrial, 2 nasopharynx, 2 gallbladder cancers, 2 lymphoma, 2 peritoneum, 1 maxillary sinus, 1 salivary gland carcinoma, 1 brain tumor, 1 leiomyosarcoma, 1 ovary cancer and 1 multiple myeloma. If reports are considered, F-18 FDG PET/CT helped to detect primary origin in 65% of these 58 patients, 38 of 54 primary sites were true positive (70%). There were 6 false positive sites (10.3%), 16 false negative (27.5%) results in F-18 FDG PET/CT. After the retrospective evaluation of the false negative patients, we have realized that primary sites were ignored in 5 of 14 patients, so actually F-18 FDG PET/CT helped in 74% of the patients showing 43 of 54 primary sites (80%). In first evaluation, F-18 FDG PET/CT missed 2 breast cancers, 1 lymphoma, 1 colon cancer and 1 intra maxillary sinus cancer. F-18 FDG PET/CT is an important imaging tool for detecting primary origin in the patients with CUP. F-18 FDG PET/CT helped in 74% of the patients showing the primary sites. In the patients with CUP, lung, breast, colon and the physiologic uptake areas should be scrutinized carefully for any tumor location.