Investigation of 2D anti-scatter grid implementation in a gantry-mounted cone beam computed tomography system for proton therapy

Background and purpose

Robust scatter mitigation by 2D anti-scatter grids (2D-ASG) in proton therapy cone beam computed tomography (CBCT) may improve target visualization and computed tomography (CT) number fidelity, allowing online dose verifications and plan adaptations. However, grid artifact-free implementation of 2D-ASG depends on the CBCT system characteristics. Thus, we investigated the feasibility of 2D-ASG implementation in a proton therapy gantry-mounted CBCT system and evaluated its impact on image quality.

Materials and methods

A prototype 2D-ASG and a grid support platform were developed for a proton therapy CBCT system with a 340 cm source to imager distance. The effect of gantry flex on 2D-ASG’s wall shadows and scan-to-scan reproducibility of 2D-ASG’s wall shadows were evaluated. Experiments were conducted to assess 2D-ASG’s wall shadow suppression and the effect of 2D-ASG on image quality.

Results

While maximum displacement in 2D-ASG wall shadows was 103 µm during gantry rotation, the drift from baseline over 3 months was 8 µm and 1 µm in the transverse and axial directions. 2D-ASG shadows were successfully suppressed in CBCT images. With 2D-ASG, maximum Hounsfield Unit (HU) nonuniformity decreased from 134 to 45 HU, contrast-to-noise ratio (CNR) increased by a factor of 2.5, and HU errors were reduced from 34 % to 5 %.

Conclusions

Proton therapy gantry flex was highly reproducible and did not noticeably affect 2D-ASG wall shadow suppression in CBCT images, supporting its feasibility in proton therapy CBCT system. Improved CT accuracy and artifact reduction with 2D-ASG could enhance CBCT-based proton therapy dose calculations.