## 3 Results  
The clinical and dosimetric characteristics of the patients included in the training and in the test set are summarised in Table 1.  
Each network training process took a mean of 24 h, while the mean generation time for a sCT study was 74 \(\pm\) 7 s. The image selection process initially performed resulted in a discard of 3128 and a selection of 1379 paired images that met the image quality criteria. A learning rate of 2\(\cdot\)10\({}^{-4}\) and a lambda value of 2000 were identified as optimal hyperparameters set during network optimisation. Figure 1 reports a comparison between the sCT obtained using the optimal hyperparameter set and the corresponding CBCT and CT for a test patient chosen as an example. \begin{table}
\begin{tabular}{c c c} \hline
**Parameters** & **Training (n = 39)** & **Test (n = 14)** \\ \hline
**Sex** & & \\ \hline Male & 20 & 7 \\ \hline Female & 19 & 7 \\ \hline
**Mean age (range)** & 64.9 (48–82) & 66.7 (49–83) \\ \hline
**Tumour** & & \\ \hline Primary & 20 & 7 \\ \hline Metastases & 19 & 7 \\ \hline
**Prescription Dose** & & \\ \hline
67.5 Gy in 25 fx & & 5 \\ \hline
55 Gy in 25 fx & & 2 \\ \hline
30 Gy in 10 fx & & 6 \\ \hline
20 Gy in 4 fx & & 1 \\ \hline \end{tabular}
\end{table} 
Table 1: Clinical and dosimetric characteristics of the patients included in the study. As for the image analysis, Figure 2 reports the values obtained in terms of MAE and ME for each single patient in the test set: average values of \(-7\)\(\pm\) 6 HU and \(36\)\(\pm\) 6 HU were obtained for ME and MAE, respectively. Palliative treatment plans were planned for patients 1-7, while curative plans were planned for patients 8-14. The mean differences of the dose DVH parameters calculated using the sCT approach with respect to those calculated on the original CT were reported as a boxplot analysis in Figure 3.  
As for PTV percentage metrics, a mean percentage difference of 0.4 \(\pm\) 0.3% was observed in the estimation of V95% [%] and 0.2 \(\pm\) 0.1% in the estimation of V105% [%]. All the DVH indicators related to dose values were in agreement within 1 Gy between the calculations on CT and sCT. Figure 4 reports the axial dose distribution obtained on a palliative case as calculated on the real CT (left) and on the synthetic CT (right). Figure 1: Example of CT (**left**), CBCT (**centre**), and synthetic CT (**right**) generated in the axial, sagittal, and coronal planes in a pelvic case. Figure 2: MAE and ME values for test cases. Patients 1–7 are palliative cases; patients 8–14 are curative cases. Dose accuracy was also evaluated in terms of gamma passing rates obtained by comparing the dose distribution calculated on the sCT with respect to the one calculated on the real CT. The results for the different gamma criteria are reported in Table 2, separately for palliative and curative test cases. No significant difference was observed between curative and palliative planning, as indicated by the \(p\)-values listed in the last column calculated using the \(t\)-test for unpaired samples. Figure 5 reports the results of the gamma passing rates obtained by merging the whole test set. Interestingly, a median gamma passing rate higher than 90% is visible even considering the tighter gamma criteria (1%/1 mm). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Curative** & **Palliative** & \(p\)**-Value** \\ \hline
1\%/1 mm & 94.7 \(\pm\) 1.2\% & 92.1 \(\pm\) 4.6\% & 0.19 \\ \hline
2\%/2 mm & 98.3 \(\pm\) 0.4\% & 97.6 \(\pm\) 1.9\% & 0.42 \\ \hline
3\%/3 mm & 99.3 \(\pm\) 0.4\% & 99.0 \(\pm\) 1.0\% & 0.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean values of gamma passing rates obtained on the test set comparing the dose distribution calculated on real and synthetic CT and considering 1%/1 mm, 2%/2 mm, and 3%/3 mm gamma as tolerance criteria. Figure 4: Visual representation of dose axial dose distribution obtained on a palliative case as calculated on the real CT (**left**) and on the synthetic CT (**right**).