Cone beam CT based validation of neural network generated synthetic CTs for radiotherapy in the head region

Purpose: In the past years, many different neural network-based conversion techniques for synthesizing computed tomographys (sCTs) from MR images have been published. While the model's performance can be checked during the training against the test set, test datasets can never represent the whole population. Conversion errors can still occur for special cases, for example, for unusual anatomical situations. Therefore, the performance of sCT conversion needs to be verified on a patient specific level, especially in the absence of a planning CT (pCT). In this study, the capability of cone-beam CTs (CBCTs) for the validation of sCTs generated by a neural network was investigated.

Methods: 41 patients with tumors in the head region were selected. 20 of them were used for model training and 10 for validation. Different implementations of CycleGAN (with/without identity and feature loss) were used to generate sCTs. The pixel (MAE, RMSE, PSNR) and geometric error (DICE, Sensitivity, Specificity) values were reported to identify the best model. VMAT plans were created for the remaining 11 patients on the pCTs. These plans were re-calculated on sCTs and CBCTs. An automatic density overriding method ( $CBC{T}_{\mathrm{RS}}$ ) and a population-based dose calculation method ( $CBC{T}_{\mathrm{Pop}}$ ) were employed for CBCT-based dose calculation. The dose distributions were analysed using 3D global gamma analysis, applying a threshold of 10% with respect to the prescribed dose. Differences in DVH metrics for the PTV and the organs-at-risk were compared among the dose distributions based on pCTs, sCTs, and CBCTs.

Results: The best model was the CycleGAN without identity and feature matching loss. Including the identity loss led to a metric decrease of 10% for DICE and a metric increase of 20-60 HU for MAE. Using the 2%/2 mm gamma criterion and pCT as reference, the mean gamma pass rates were 99.0 $\pm$ 0.4% for sCTs. Mean gamma pass rate values comparing pCT and CBCT were 99.0 $\pm$ 0.8% and 99.1 $\pm$ 0.8% for the $CBC{T}_{\mathrm{RS}}$ and $CBC{T}_{\mathrm{Pop}}$ , respectively. The mean gamma pass rates comparing sCT and CBCT resulted in 98.4 $\pm$ 1.6% and 99.2 $\pm$ 0.6% for $CBC{T}_{\mathrm{RS}}$ and $CBC{T}_{\mathrm{Pop}}$ , respectively. The differences between the gamma-pass-rates of the sCT and two CBCT-based methods were not significant. The majority of deviations of the investigated DVH metrices between sCTs and CBCTs were within 2%.

Conclusion: The dosimetric results demonstrate good agreement between sCT, CBCT, and pCT based calculations. A properly applied CBCT conversion method can serve as a tool for quality assurance procedures in an MR only radiotherapy workflow for head patients. Dosimetric deviations of DVH metrics between sCT and CBCTs of larger than 2% should be followed up. A systematic shift of approximately 1% should be taken into account when using the $CBC{T}_{\mathrm{RS}}$ approach in an MR only workflow.