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

Simple Summary Daily adaptive proton therapy will allow the unique properties of protons to be fully exploited. Cone-beam CT (CBCT) is the primary imaging modality considered for daily adaptation due to its low cost, compactness, and thus wide availability. However, there are proton therapy centers equipped with CT-on-rails or considering the installation of such scanners for the so-called "near-treatment-position" imaging. Our study addresses the critical question, whether CT-on-rails is a suitable modality for daily adaptive proton therapy. Although high precision accuracies have been claimed for CT-on-rails, no quantitative study of the adaptation efficacy with increased treatment execution uncertainties has ever been performed. In this paper, we demonstrate that the expected uncertainties will not affect the dosimetric efficacy of the adaptation based on in-room CT for head and neck cancers, and thus CT-on-rails applied to "near-treatment-position" imaging is a suitable modality for online adaptive proton therapy. 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 sigma. It is a combination of the isocenter-matching sigma(i) and random patient movements induced by the couch motion sigma(m). The former is assumed to never exceed 1 mm. For the latter, we studied three different scenarios with sigma(m) = 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 sigma(m) = 1 and 2 mm. Offsets with sigma(m) = 3 mm resulted in underdosage to CTV and hot spots of considerable volume. Conclusion: Since sigma(m) 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.