18F-FDG PET during stereotactic body radiotherapy for stage I lung tumours cannot predict outcome: a pilot study

(18)F-Fluorodeoxyglucose positron emission tomography (FDG PET) has been used to assess metabolic response several months after stereotactic body radiotherapy (SBRT) for early-stage non-small cell lung cancer. However, whether a metabolic response can be observed already during treatment and thus can be used to predict treatment outcome is undetermined. Ten medically inoperable patients with FDG PET-positive lung tumours were included. SBRT consisted of three fractions of 20 Gy delivered at the 80% isodose at days 1, 6 and 11. FDG PET was performed before, on day 6 immediately prior to administration of the second fraction of SBRT and 12 weeks after completion of SBRT. Tumour metabolism was assessed semi-quantitatively using the maximum standardized uptake value (SUV(max)) and SUV(70%). After the first fraction, median SUV(max) increased from 6.7 to 8.1 (p = 0.07) and median SUV(70%) increased from 5.7 to 7.1 (p = 0.05). At 12 weeks, both median SUV(max) and median SUV(70%) decreased by 63% to 3.1 (p = 0.008) and to 2.5 (p = 0.008), respectively. SUV increased during treatment, possibly due to radiation-induced inflammation. Therefore, it is unlikely that (18)F-FDG PET during SBRT will predict treatment success