Predictive value of interim ¹⁸F-FDG-PET in patients with non-small cell lung cancer treated with definitive radiation therapy

Purpose We evaluated that early metabolic response determined by ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) during radiotherapy (RT), predicts outcomes in non-small cell lung cancer. Material and methods Twenty-eight patients evaluated using pretreatment ¹⁸F-FDG-PET/CT (PET_pre) and interim ¹⁸F-FDG-PET/CT (PET_interim) after 11 fractions of RT were retrospectively reviewed. Maximum standardized uptake value (SUV_max) was calculated for primary lesion. Predictive value of gross tumor volume (ΔGTV) and SUV_max (ΔSUV_max) changes was evaluated for locoregional control (LRC), distant failure (DF), and overall survival (OS). Metabolic responders were patients with ΔSUV_max >40%. Results Metabolic responders showed better trends in 1-year LRC (90.9%) than non-responders (47.1%) (p = 0.086). Patients with large GTV_pre (≥120 cc) demonstrated poor LRC (hazard ratio 4.14, p = 0.022), while metabolic non-responders with small GTV_pre (<120 cc) and metabolic responders with large GTV_pre both had 1-year LRC rates of 75.0%. Reduction of 25% in GTV was not associated with LRC; however, metabolic responders without a GTV response showed better 1-year LRC (83.3%) than metabolic non-responders with a reduction in GTV (42.9%). Metabolic responders showed lower 1-year DF (16.7%) than non-responders (50.0%) (p = 0.025). An ΔSUV_max threshold of 40% yielded accuracy of 64% for predicting LRC, 75% for DF, and 54% for OS. However, ΔGTV > 25% demonstrated inferior diagnostic values than metabolic response. Conclusions Changes in tumor metabolism diagnosed using PET_interim during RT better predicted treatment responses, recurrences, and prognosis than other factors historically used.