Purpose: To evaluate clinical impacts by couch moving rails on proton dose delivery in sarcoma cancer treatment, dose errors on critical structures were simulated and measured with different positions of the rails.
Methods: Spot delivery to the patient body in a treatment room can be reproduced with different lateral positions of moving rails according to patient setup variations. Including patient anatomy and different positions of moving rails, computed tomography images were reconstructed and used to investigate dose errors caused by spots passing through moving rails. Dosimetric effects on target volumes and organs-at-risk were analyzed with dose-volume-histograms and clinically significant parameters. To demonstrate effects on proton beam characteristics, range pullbacks and skewness and sizes in spot profiles were evaluated with range measurement in water and a scintillator detector. Differences in gamma passing rates and dose distributions were evaluated with the measurement with an ion-chamber array. To avoid spot interference caused by moving rails, a different configuration of beam angles and strategies of dose optimization were devised.
Results: In dose simulation, spots interfered by the moving rails showed underdose for target volume and OAR dose change by 3-5%. It resulted in a difference of 7 mm in range and 5 mm in spot size. Using different isocenter positions and beam angles could improve dose conformity and reduce dose degradation by spot interference.
Conclusion: Spot interference by moving rails depending on patient setup variations showed distinct errors on proton beam range and spot profiles. Changes in proton beam characteristics in delivery using pencil beam scanning brought out clinical impacts on dose for critical structures. A better planning strategy and dose simulation using different positions of moving rails could reduce spot interference errors in proton treatment using pencil beam scanning.