Objective: The aim of the present study was to evaluate the clinical significance of the post-radiotherapy 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) response for detecting residual disease and predicting survival outcome in patients with nasopharyngeal cancer.
Methods: We reviewed 143 patients with nasopharyngeal cancer who underwent 18F-FDG PET within 6 months after completion of radiotherapy between 2001 and 2012. 18F-FDG PET findings at the primary tumor (T-) and regional lymph nodes (N-) were separately assessed and considered negative [PET (-)] or positive [PET (+)] depending on the remaining focal increased uptake of 18F-FDG that was greater than that of the surrounding muscle or blood vessels. The standard of reference was histopathological confirmation or clinical/imaging follow-up. Overall survival (OS), distant metastasis-free survival (DMFS), and locoregional recurrence-free survival (LRRFS) rates were estimated from the date of the start of radiotherapy.
Results: The median follow-up period was 73 months (range, 9-182 months). Overall, 83 and 66% of patients achieved T-PET (-) and N-PET (-) responses, and the negative-predictive values (NPVs) for T- and N- were 100 and 99%, respectively. The sensitivity, specificity, and positive-predictive value were 100, 84, and 8% for T-, and 67, 80, and 7% for N-, respectively. The 5-year OS, DMFS, and LRRFS rates were 83, 83, and 87%, respectively, and patients with N-PET (+) with SUVmax >2.5 showed significantly inferior 5-year OS and DMFS rates than patients with N-PET (-) or N-PET (+) with SUVmax ≤2.5 (44 vs 86%, p = 0.004; 36 vs 85%, p < 0.001).
Conclusion: In patients that have received definitive (chemo)radiotherapy for nasopharyngeal cancer, 18F-FDG PET within 6 months of completion of treatment has a high NPV for predicting residual disease and is prognostic for long-term treatment outcomes. Patients with remaining focal increased uptake of 18F-FDG at lymph nodes may benefit from more aggressive treatments, and further studies are needed to validate the clinical significance of post-radiotherapy 18F-FDG PET.
Advances in knowledge: We found that post-radiotherapy 18F-FDG PET findings have a high NPV for detecting residual disease and are a significant prognostic factor for treatment outcomes.