Half-Brain Delineation for Prediction of Radiation-Induced Temporal Lobe Injury in Nasopharyngeal Carcinoma Receiving Intensity-Modulated Radiotherapy

PURPOSE: To investigate the role of half-brain delineation in the prediction of radiation-induced temporal lobe injury (TLI) in nasopharyngeal carcinoma (NPC) receiving intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: A total of 220 NPC cases treated with IMRT and concurrent platinum-based chemotherapy were retrospectively analyzed. Dosimetric parameters of temporal lobes, half-brains, and brains included maximum dose (D(max)), doses covering certain volume (D(V)) from 0.03 to 20 cc and absolute volumes receiving specific dose (V(D)) from 40 to 80 Gy. Inter-structure variability was assessed by coefficients of variation (CV) and paired samples t-tests. Receiver operating characteristic curve (ROC) and Youden index were used for screening dosimetric parameters to predict TLI. Dose/volume response curve was calculated using the logistic dose/volume response model. RESULTS: CVs of brains, left/right half-brains, and left/right temporal lobes were 9.72%, 9.96%, 9.77%, 27.85%, and 28.34%, respectively. Each D(V) in temporal lobe was significantly smaller than that in half-brain (P < 0.001), and the reduction ranged from 3.10% to 45.98%. The area under the curve (AUC) of D(V) and V(D) showed an “increase-maximum-decline” behavior with a peak as the volume or dose increased. The maximal AUCs of D(V)s in brain, half-brain and temporal lobe were 0.808 (D(2cc)), 0.828 (D(1.2cc)) and 0.806 (D(0.6cc)), respectively, and the maximal AUCs of V(D)s were 0.818 (D(75Gy)), 0.834 (V(72Gy)) and 0.814 (V(70Gy)), respectively. The cutoffs of V(70Gy) (0.86 cc), V(71Gy) (0.72 cc), V(72Gy) (0.60 cc), and V(73Gy) (0.45 cc) in half-brain had better Youden index. TD5/5 and TD50/5 of D(1.2cc) were 58.7 and 80.0 Gy, respectively. The probability of TLI was higher than >13% when V(72Gy)>0 cc, and equal to 50% when V(72Gy) = 7.66 cc. CONCLUSION: Half-brain delineation is a convenient and stable method which could reduce contouring variation and could be used in NPC patients. D(1.2cc) and V(72Gy) of half-brain are feasible for TLI prediction model. The dose below 70 Gy may be relatively safe for half-brain. The cutoff points of V(70–73Gy) could be considered when the high dose is inevitable.