Characterization of FDG-PET images after stereotactic body radiation therapy for lung cancer

Background and purpose: The purpose was to characterize 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET) findings after stereotactic body radiation therapy (SBRT) for lung cancer. Materials and methods: This was a retrospective review of 32 FDG-PET scans from 23 patients who underwent SBRT for lung cancer and who showed no evidence of local recurrence. The FDG uptake by lesions was assessed visually using a 3-point scale (0, none or faint; 1, mild; or 2, moderate to intense), and the demarcation (ill- or well-defined) was evaluated. For semi-quantitative analysis, the maximum standardized uptake value (SUVmax) was calculated. Results: Grade 2 intensity was observed in 70%, 33%, 30%, and 0% of PET scans performed <6, 6-12, 12-24, and >24 months, respectively, after SBRT; well-defined demarcation was observed in 80%, 33%, 40%, and 17%, respectively, and the respective means of the SUVmax were 4.9, 2.6, 3.0, and 2.3. The SUVmax was significantly higher for scans performed at <6 months than at 6-12 or >24 months. Conclusions: FDG uptake tended to be intense and well-defined at early times after SBRT, especially within 6 months, and was faint and ill-defined at later periods. Moderate to intense FDG uptake observed soon after SBRT does not always indicate a residual tumour.