Synthetic CT Generation from CBCT Using Deep Learning for Adaptive Radiotherapy in Prostate Cancer

Mustafa Çağlar^1*Kerime Selin Ertaş²Mehmet Sıddık Cebe²Navid Kheradmand²Evrim Metcalfe²

• ¹Istanbul Medipol University, Istanbul, Türkiye
• ²Istanbul Medipol Universitesi, Fatih, Türkiye

Objective: In this study, the accuracy of deep learning-based models developed for synthetic CT (sCT) generation from conventional Cone Beam Computed Tomography (CBCT) images of prostate cancer patients was evaluated. The clinical applicability of these sCTs in treatment planning and their potential to support adaptive radiotherapy decision-making were also investigated. Methods: A total of 50 CBCT-CT mappings were obtained for each of 10 retrospectively selected prostate cancer patients, including one planning CT (pCT) and five CBCT scans taken on different days during the treatment process. All images were preprocessed, anatomically matched and used as input to the U-Net and ResU-Net models trained with PyTorch after z-score normalisation. The sCT outputs obtained from model outputs were quantitatively compared with the pCT with metrics such as SSIM, PSNR, MAE, and HU difference distribution. Results: Both models produced sCT images with higher similarity to pCT compared to CBCT images. The mean SSIM value was 0.763±0.040 for CBCT-CT matches, 0.840±0.026 with U-Net and 0.851±0.026 with ResU-Net, with a significant increase in both models (p<0.05). PSNR values were 21.55 ± 1.38 dB for CBCT, 24.74±1.83 dB for U-Net, and 25.24±1.61 dB for ResU-Net. ResU-Net provided a statistically significant higher PSNR value compared to U-Net (p<0.05). In terms of MAE, while the mean error in CBCT-CT matches was 75.2±18.7 HU, the U-Net model reduced this value to 65.3±14.8 HU and ResU-Net to 61.8±13.7 HU (p<0.05). Conclusion: Deep learning models trained with simple architectures such as U-Net and ResU-Net provide effective and feasible solutions for the generation of clinically relevant sCT from CBCT images, supporting accurate dose calculation and facilitating adaptive radiotherapy workflows in prostate cancer management.