Evaluation of the availability of single‐position treatment with a rotating gantry and the validity of deformable image registration dose assessment for pancreatic cancer carbon‐ion radiotherapy

Background This study aimed to evaluate the clinical acceptability of rotational gantry‐based single‐position carbon‐ion radiotherapy (CIRT) to reduce the gastrointestinal (GI) dose in pancreatic cancer. We also evaluated the usefulness of the deformable image registration (DIR)‐based dosimetry method for CIRT. Material and methods Fifteen patients with pancreatic cancer were analyzed. The treatment plans were developed for four beam angles in the supine (SP plan) and prone (PR plan) positions. In the case of using multiple positions, the treatment plan was created with two angles for each of the supine and prone position (SP + PR plan). Dose evaluation for multiple positions was performed in two ways: by directly adding the values of the DVH parameters for each position treatment plan (DVH sum), and by calculating the DVH parameters from the accumulative dose distribution created using DIR (DIR sum). The D 2cc and D 6cc of the stomach and duodenum were recorded for each treatment plan and dosimetry method and compared. Results There were no significant differences among any of the treatment planning and dosimetry methods ( p > 0.05). The DVH parameters for the stomach and duodenum were higher in the PR plan and SP plan, respectively, and DVH sum tended to be between the SP and PR plans. DVH sum and DIR sum, DVH sum tended to be higher for D 2cc and DIR sum tended to be higher for D 6cc . Conclusion There were no significant differences in the GI dose, which suggests that treatment with a simple workflow performed in one position should be clinically acceptable. In CIRT, DIR‐based dosimetry should be carefully considered because of the potential for increased uncertainty due to the steep dose distributions.