CBCT-based synthetic CT generation using a hybrid approach for adaptive radiotherapy of nasopharyngeal carcinoma

Abstract Objective: This study aims to utilize a hybrid approach of phantom correction and deep learning for synthesized CT (sCT) images generation based on cone-beam CT (CBCT) images for nasopharyngeal carcinoma (NPC). Methods: A total of 52 CBCT/CT paired images of NPC patients were used for training (41), validation (11) datasets. Hounsfield Units (HU) of the CBCT images was corrected by a commercial CIRS phantom. Then the original CBCT and the corrected CBCT (CBCT_cor) were trained separately with the some cycle generative adversarial network (CycleGAN) to generate SCT1 and SCT2. The mean error (ME) and mean absolute error (MAE) were used to quantify the image quality. For the patients in the validation datasets, the contours and treatment plans in CT images were transferred to original CBCT, CBCT_cor, SCT1 and SCT2 for dosimetric comparison. Finally, dose distribution, dosimetric parameters and 3D gamma pass rate were analyzed. Results: Compared with rigidly registered CT (RCT), the MAE of CBCT, CBCT_cor, SCT1 and SCT2 were 346.11 ± 13.58HU, 145.95 ± 17.64HU, 105.62 ± 16.08HU and 83.51 ± 7.71HU, respectively. Moreover, the average dosimetric parameter differences for the CBCT_cor, SCT1 and SCT2 were 2.7% ± 1.4%, 1.2% ± 1.0% and 0.6% ± 0.6%, respectively. Using the dose distribution of RCT images as reference, the 3D gamma pass rate of the hybrid method was significantly better than the other methods. Conclusion: A novel hybrid approach based on HU-ED correction and CycleGAN was developed to generate sCT images for CBCT images of NPC patients. The image quality and dose accuracy of the hybrid approach were outperform the simple CycleGAN method. This finding has great significance for the clinical application of adaptive radiotherapy for NPC.