Collimator and Detector Optimization Towards Real-Time Motion Management In Lung SBRT with Compton Scatter Detection
Abstract
Purpose
During lung SBRT, tumor motion is managed using 4DCT-based ITV expansion, respiratory or external gating, or breath-hold techniques, each with limitations. Existing real-time tracking methods require implanted fiducials or additional imaging dose. A non-invasive, dose-free method for real-time tumor tracking is warranted. Passive detection of Compton-scattered, therapeutic photons offers a novel solution. A comprehensive Monte Carlo (MC) framework was utilized to optimize the collimator and detector towards motion management with Compton scatter detection.
Methods
The validated GEANT4 Application for Tomographic Emission (GATE) MC toolkit, which allows accurate scintillation system modelling, was utilized. Phase-space analyses first determined the optimal angular acceptance range (AAR) of scattered photons (SPs) that preserved geometry of a layered cuboid lung tumor phantom, selected for its discernability in novel imaging development, for a clinical 6 MV FFF beam. The detector position, scintillation material, and collimator properties: septal height (H), thickness (T), interspace (I), and material of parallel-focused 2D anti-scatter grids (ASGs) were varied to preferentially accept SPs within the determined optimal AAR. The angular detection significance metric (ADSM), defined as the number of detected SPs within the optimal AAR divided by the square root of the total detected photons, served as the performance metric.
Results
The phantom image was geometrically accurate when the SPs’ AAR was restricted to ±0.5° about the detector position (e.g., 89.5–90.5° for a detector at 90°). The highest ADSM value (15.4), corresponding to the optimal design, was achieved using a tungsten ASG with H, T, and I of 57, 0.2, and 1.0 mm, respectively, positioned at 45° relative to the treatment head (0°) and coupled to a gadolinium oxysulfide (Gd₂O₂S:Tb) scintillation crystal.
Conclusion
MC modelling demonstrated that optimizing the detector and ASG design can enhance SP selection, preserve geometric fidelity, and produce images toward Compton scatter–based motion management for lung SBRT.