BACKGROUND

Magnetic Resonance (MR)-based synthetic Computed Tomography (sCT) generation is an emerging promising technique, required for the transition from conventional planning workflows to MR-only radiotherapy planning. This shift aims to replace CT acquisition with a sCT improving both cost efficiency and burden to the patient. Generative Adversarial Networks (GANs) have shown some of the best performance in this area.

PURPOSE

This study aims to identify optimal approaches to improve the quality and clinical applicability of MR-based sCT generation for treatment planning by performing an extensive comparison of GAN architectures and parameters thereof. It focuses on the abdominal region, which still lacks certified medical products for sCT generation.

METHODS

In order to improve the current state of deep learning technologies, we generated sCTs based on abdominal MR images of 154 cancer patients using GANs, varying the following parameters: (1) generator architectures (U-Net, ResNet); (2) GAN architectures trained in paired (Pix2Pix) and unpaired fashion (CycleGAN and CUT); (3) number of input-output channels (2D, 2.5D); (4) training set size. The quality of sCT generation was assessed by using both image similarity and dosimetric metrics; correlation between the two was evaluated. The dosimetric accuracy was evaluated through an automated process that compared the dose distributions of photon treatment plans calculated on sCT and CT images, using Dose-Volume Histogram (DVH) parameters for tumor and organs at risk.

RESULTS

The Pix2Pix model, trained in paired fashion with 2.5D input-output channels and a ResNet generator emerged as the best-performing model, achieving a mean absolute error (MAE, mean) of 63.21 HU, a planning target volume Dmean difference of -0.09%, and no outliers above 2% for other DVH parameters. This configuration addressed prior challenges of Pix2Pix with bone and rigid organ boundary generation, delivering robust results even for cases with significant air pockets. The 2D input-output channel configuration showed beneficial for GANs trained in unpaired fashion, achieving a mean MAE of 66.97 HU for CycleGAN and 69.49 HU for CUT. Both delivered clinically applicable results, with mean DVH discrepancies below 0.8%. Expanding the training set size was essential for minimizing outliers in dosimetric parameters. High correlation was observed between the image similarity metrics-MAE, MAE bones, structural similarity index measure-and target DVH parameters, with Pearson coefficients ranging from 0.77 to 0.9. However, within the clinically relevant range of DVH deviations (± 2%), stochastic variations obscured linear trends.

CONCLUSIONS

The study provided a new benchmark for the abdominal sCT generation task, showing its clinical applicability for treatment planning and further advancing the state-of-the-art. This study also confirmed that image similarity metrics alone can not reliably predict small dosimetric deviations within a clinical threshold; but contributed by identifying specific metrics that correlate with DVH discrepancies above ± 5%, offering valuable tools for training, evaluation, and standardization of reporting across studies.