Assessment of nasopharyngeal carcinomas after radiotherapy with a hybrid camera-based PET/CT system using 18F-FDG

Objective: To evaluate the clinical values of hybrid imaging using a gamma camera-based PET/CT system and 18F-FDG in detection of recurrent or persistent nasopharyngeal carcinomas after radiotherapy. Methods: Nineteen patients with nasopharyngeal carcinomas were evaluated three months after radiotherapy. The recurrent or metastasis lesions were suspected by routinely diagnostic X-ray CT examinations. Hybrid imaging was performed using a device combining low-dose CT and gamma camera-based PET system (GE Millennium VG with Hawkeye) to differentiate recurrent or residual tumors from benign lesions. The presence or absence of recurrent or metastasis lesions were confirmed by the histopathologic examinations and/or 3-12 month clinical follow-up. The sequence of x-ray transmission scan and coincidence detection was initiated 60 min after the injection of 158.4-314.5 MBq (4.28-8.5 mCi) 18F-FDG. The data are rebinned into 90 projections and reconstructed iteratively including attenuation correction. The resulting images were interpreted by at least three nuclear medicine physicians together. The radioactivity ratios of target to non-target (T/NT) were calculated. Results: In the 10 patients with proven recurrent or metastasis lesions, CT findings were probable in 3 patients and doubtful in 7 ones. 18F-FDG coincidence imaging were true positive and showed significantly increased FDG uptake in all the 10 patients (10/10). The lesions' T/NT were between 1.23-5.75 (3.25±1.07 mean±s.d.). In the 9 patients eliminated the tumor recurrence, CT findings were probable in 2 patients and doubtful in 7 patients. 18F-FDG coincidence imaging were tree negative in 8 patients (8/9). The lesions' T/NT were between 0.87-3.21 (1.25±0.49 mean±s.d.). Only 1 finding was false positive in the patient with infection in the pharyngeal back and neck lymph nodes. The lesions' T/NT was 3.21. No matter what the 18F-FDG coincidence imaging was positive or negative, the fusion image can more directly and apparently show the metabolic activity in the suspected lesions at the parapharyngeal space, especially in the pharyngeal back and the base of the skull. Conclusion: Hybrid imaging using a gamma camera-based PET/CT system and 18F-FDG may improve the diagnostic accuracy in detection of recurrent or persistent nasopharyngeal carcinomas after radiotherapy. It appears to provide a reliable diagnostic tool for clinical management of theses patients. (authors)