2275 - A Preliminary Study of Gated Stereotactic Body Radiation Therapy Utilizing the Beam-Pause Function of a Surface Guidance System

Purpose/Objective(s): Respiratory tumor motion has been traditionally managed mainly in three ways: enclosing ITV, breath hold, and gating. Over the past decade, surface guidance (SG) systems have improved daily patient setup and intrafraction motion monitoring. Most SG providers offer a beam-pause function when monitored area moves beyond a set tolerance, catching unexpected motion and increasing confidence of breath hold treatment. Yet, gating function hasn’t been made widely available, if at all. This study explores the feasibility of using an SG beam-pause feature as a surrogate gating mechanism for lung SBRT.

Materials/Methods: Data was collected from a motorized lung motion phantom. The phantom has an acrylic body and a cylindrical wooden ‘left lung’ which can move in the sup-inf direction to simulate respiratory motion. The lung module has an embedded off-center ‘solid tumor’ (15 mm radius sphere) and can be rotated to introduce axial motion. The phantom was set to breath at 12 rpm, tumor oscillation of 4 cm in the sup-inf direction, with or without axial motion. 4DCT was acquired for each motion pattern, and 10 amplitude-based phases were created respectively. Three types of average CT were generated: all phases for free breathing, 3-phase and 5-phase at the exhale end for a narrow and a relaxed gating. GTVs were contoured on each selected phase to form an ITV, then expanded 5 mm for the PTV. Six lung SBRT plans (50 Gy in 5 fractions, 6FFF) were created. The phantom has an ‘surface’ structure that oscillates 1cm vertically in rhythm with the lung module. This structure was contoured in each phase, and the offset between the treating phase and reference (exhale-end) phase was measured to yield the beam-release criteria for SG. Because the SG system imposes a 0.5 s beam-on delay, phantom breathing was slowed to 4 rpm during delivery to reduce the weight of the delay. Dose was measured at the tumor center using an ionization chamber (IC) and EBT4 film.

Results: The GTV volumes ranged from 11.7 to 15.7 cc across all phases of the two motion patterns. SG-detected ‘surface’ motion was 9-9.5 mm. For the 3-phase and 5-phase gating, motion threshold of 2 mm and 5 mm were selected. Detailed data are provided in the table.

Conclusion: This preliminary study demonstrates a workflow using the SG beam-pause function as a surrogate gating technique for lung SBRT. The SBRT plans were delivered successfully, and the gated delivery showed good agreement between measured and planned doses. Ongoing efforts will assess performance under more realistic motion patterns and further evaluate SG detection limitation and reliability.