*Article* **Synthetic CT in Carbon Ion Radiotherapy of the Abdominal Site**

**Giovanni Parrella 1,\*, Alessandro Vai 2, Anestis Nakas 1, Noemi Garau 1, Giorgia Meschini 1, Francesca Camagni 1, Silvia Molinelli 2, Amelia Barcellini 3,4, Andrea Pella 5, Mario Ciocca 2, Viviana Vitolo 3, Ester Orlandi 6, Chiara Paganelli <sup>1</sup> and Guido Baroni <sup>1</sup>**


**Abstract:** The generation of synthetic CT for carbon ion radiotherapy (CIRT) applications is challenging, since high accuracy is required in treatment planning and delivery, especially in an anatomical site as complex as the abdomen. Thirty-nine abdominal MRI-CT volume pairs were collected and a three-channel cGAN (accounting for air, bones, soft tissues) was used to generate sCTs. The network was tested on five held-out MRI volumes for two scenarios: (i) a CT-based segmentation of the MRI channels, to assess the quality of sCTs and (ii) an MRI manual segmentation, to simulate an MRI-only treatment scenario. The sCTs were evaluated by means of similarity metrics (e.g., mean absolute error, MAE) and geometrical criteria (e.g., dice coefficient). Recalculated CIRT plans were evaluated through dose volume histogram, gamma analysis and range shift analysis. The CT-based test set presented optimal MAE on bones (86.03 ± 10.76 HU), soft tissues (55.39 ± 3.41 HU) and air (54.42 ± 11.48 HU). Higher values were obtained from the MRI-only test set (MAEBONE = 154.87 ± 22.90 HU). The global gamma pass rate reached 94.88 ± 4.9% with 3%/3 mm, while the range shift reached a median (IQR) of 0.98 (3.64) mm. The three-channel cGAN can generate acceptable abdominal sCTs and allow for CIRT dose recalculations comparable to the clinical plans.

**Keywords:** synthetic CT; MRI guidance; MRI-only; image-guided radiotherapy; carbon ion radiotherapy; particle therapy; deep learning
