2189 - A Treatment Planning Study of a New Fixed Beamline Model for an Upright Compact Proton Therapy System

Purpose/Objective(s): Large gantry systems are a major barrier to the widespread adoption of proton therapy due to footprint and cost. This has led to increased interest in fixed proton beamlines utilizing upright patient positioners. Assessing treatment plan quality for these systems is crucial. This study aims to determine whether clinically acceptable plans can be achieved for several treatment sites using a new fixed beamline, upright proton therapy system.

Materials/Methods: Treatment datasets were collected for five prostate cases, five breast cases, one head-and-neck case, and one liver case, all of which were previously treated using a proton gantry system. The datasets included CT images in supine positions, clinical contours, beam geometries, and treatment prescriptions. Each case was re-planned using the new fixed beamline model in treatment planning system, utilizing only coplanar beams in the axial plane. The patient positioner back angles were adjusted between 0 and 15 degrees for some cases to evaluate the dosimetric differences. The previously delivered plans served as a reference to identify any deviations from clinical prescription goals. However, no direct comparison between the original and new plans was conducted. Additionally, potential organ deformations due to the change in patient position (sitting vs. lying) were not considered in this study. New plans were evaluated according to the clinical prescription.

Results: In five prostate cases, every clinical goal was successfully met. In three of five breast cases, the target dose prescription was achieved, including lymph node irradiation, without violating clinical goals for heart, ipsilateral lung, left anterior descending artery (LAD), and skin doses. Two of the breast cases with thin chest wall targets required multiple tangential beams instead of the standard, single en face beam to achieve the required dose for internal mammary nodes. In the head-and-neck case, the original plan utilized a non-coplanar vertex field and 3 coplanar beams to spare the right temporal lobe, whereas the fixed beamline model utilized a 6 coplanar beams to spare the temporal lobe and skin while maintaining target coverage and dose limits to the eye. In the liver case, a clinically acceptable plan was achieved

Conclusion: The proton beam model for a novel upright proton therapy device successfully generated high-quality plans in a variety of treatment sites studied. This study indicates the dosimetric feasibility of the new single-room compact proton system. Further analysis, including plan robustness and the effects of motion and organ deformation in upright positions, should be conducted for clinical evaluation.