Free-Breathing–Gated Delivery Can Achieve High-Quality Proton Treatment for Lung Cancer Patients
Abstract
Purpose
While 30% of lung cancer patients cannot tolerate breath-hold, free-breathing(FB) proton pencil-beam-scanning(PBS) deliveries compromise target coverage from the interplay effects of large motion. This study presents a novel FB-gated spirometer-based respiratory monitored PBS delivery for maintaining target coverage and reducing organ-at-risk(OAR) dose with existing cone-beam(CBCT) based image-guidance(IGRT) and establishes its treatment planning workflow.
Methods
A motion-evaluation tool was developed for variable gating phases that assessed motion and water-equivalent-thickness(WET) variation using MATLAB-based OpenREGGUI. Target contours(iCTV), partial-phase CT, and treatment plan were created for an example patient. As a surrogate for average CBCT to confirm reproducibility of IGRT with full-phase target while delivering over a partial-phase target, retrospectively, image registration quality of partial-phase target and full-phase target was quantified comparing simulation 4DCT scan and verification scan(Vscan). End-to-end testing was performed with a 4D-motion phantom, including confirmation of delivery within the gating window.
Results
Comparison of 60Gy-total plans for the example patient with full-phase and 0-20%-phase CT showed decrease in target volume(iCTV:108cc→97cc) with increased Lung-GTV volume(3060cc→3200cc) resulting in equivalent target coverage(D95%=60Gy) and decreased OAR dose to spinal cord(Dmax:17Gy→16.5Gy), lung(V20Gy:5.2%→5.0%), and heart(Dmean:5.3Gy→5.2Gy). WET-variation for 0-20%-phase was 0.98 for MDA, mean-distance-to-agreement, and DSC, dice-similarity-coefficient, with HU histogram overlap of >0.96). A plan delivered on the phantom, received irradiation only within the gating window, including during irregular physiological breathing. Delivered dose during hyperventilation, and short-breath-hold agreed with planned dose (3%/3mm gamma-pass-rates of 94.9% and 97.6%).
Conclusion
FB-gated delivery using CBCT for IGRT was demonstrated via planning on an example patient and end-to-end testing with a 4D-motion phantom. The technique may expand the lung cancer patient cohort eligible for particle therapy. Further validation is warranted with a larger cohort of retrospective patients.