CT-on-Rails Versus In-Room CBCT for Online Daily Adaptive Proton Therapy of Head-and-Neck Cancers.

Purpose: To compare the efficacy of CT-on-rails versus in-room CBCT for daily adaptive proton therapy.

Methods: We analyzed a cohort of ten head-and-neck patients with daily CBCT and corresponding virtual CT images. The necessity of moving the patient after a CT scan is the most significant difference in the adaptation workflow, leading to an increased treatment execution uncertainty . It is a combination of the isocenter-matching and random patient movements induced by the couch motion . The former is assumed to never exceed 1 mm. For the latter, we studied three different scenarios with = 1, 2, and 3 mm. Accordingly, to mimic the adaptation workflow with CT-on-rails, we introduced random offsets after Monte-Carlo-based adaptation but before delivery of the adapted plan.

Results: There were no significant differences in accumulated dose-volume histograms and dose distributions for = 1 and 2 mm. Offsets with = 3 mm resulted in underdosage to CTV and hot spots of considerable volume.

Conclusion: Since typically does not exceed 2 mm for in-room CT, there is no clinically significant dosimetric difference between the two modalities for online adaptive therapy of head-and-neck patients. Therefore, in-room CT-on-rails can be considered a good alternative to CBCT for adaptive proton therapy.