Detection of unknown primary tumors using whole body FDG PET

Abstract

Objective: To assess the usefulness of 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) in locating occult primary lesions.Methods: 50 patients with varying heterogeneous metastases of unknown primary origin were referred for FDG PET. The locations of the known metastatic tumor manifestations were distributed as follows: cervical lymph nodes metastases (n=18), skeletal metastases (n=15), cerebral metastases (n=12), others (n=5). All patients underwent whole body¹⁸F-FDG PET imaging. The images were interpreted by visual inspection and semi-quantitative analysis (standardized uptake value, SUV). The patients had undergone conventional imaging within 2 weeks of FDG PET. Surgical, clinical and histopathologic findings were used to assess the performance of FDG PET.Results: FDG PET was able to detect the location of the primary tumor in 32/50 patients (64%). The primary tumors were proved by histopathologic results, and located in the lungs (n=17), the nasopharynx (n=9), the breast (n=2), the ovary (n=1), the colon (n=1), the prostate (n=1), the thyroid (n=1). FDG PET were proved false positive in 2 patients (4%), and the suspicious primary tumors were in uterus and colon respectively. During the clinical follow-up of 2 to 26 months, the primary tumor was found in only 2 patients (prostate cancer, gastric cancer).Conclusion: PET imaging allows identification of the primary site and metastatic lesions(including bone and soft tissue metastases) at a single examination. Whole body¹⁸F-FDG PET allows effective localization of the unknown primary site of origin and can contribute substantially to patient care.