Dosimetric study of Qfix kVue rails for pencil beam scanning proton beam

Background: The Qfix kVue table is widely used in proton beam therapy. It is made with light weight carbon fiber material in order to minimize its water-equivalent thickness. However, the design uses two rails underneath the couch in order to support the weight of the patient. These rails are structured frames made of high density carbon fiber with larger water-equivalent thickness. Moreover, depending on the relative orientation between the rail structures and the incident beam angles, the water-equivalent thickness can change significantly. Common clinical practice assumes that the rails are out of the beam path for planning and relies on the treatment staff to ensure that the rails are in the designated position relative to the patient during actual treatment. This study examines the dosimetric effects of the rails. Methods: A Qfix kVue table with rails was scanned using computed tomography (CT) and proton pencil beam scanning plans were generated with and without rails in the beam path. The impact of the rails on the proton range and dose distribution was studied in a virtual water tank and a test patient. Results: The water-equivalent thickness of the rails varies from a few millimeters to a few centimeters, depending on the beam angle and the location. In general, range of the proton beam is reduced most significantly and the dose disturbed most drastically when the beam traverses parallel through the outer and/or inner edges of the rails. Conclusions: Rails have a sizable and non-uniform impact on the dose distribution. They should be kept out of the beam path during treatment.