Improvement of Soft Tissue Contrast and CT Number Accuracy in Proton Therapy CBCT With Hardware-based Scatter Suppression: Results From a Prospective Trial.

PURPOSE: Adaptive proton therapy using cone beam CT (CBCT) requires accurate CT numbers for dose calculation and sufficient soft tissue visibility for reliable target and organ delineation. However, scatter in conventional CBCT causes substantial CT number inaccuracies and poor soft tissue contrast, limiting its use in adaptive workflows. To address this, a novel 2-dimensional antiscatter grid (2D ASG) prototype and raw data correction algorithms were developed for a proton gantry-mounted CBCT system. This prospective study evaluated their impact on CBCT image quality in human subjects.

METHODS AND MATERIALS: Ten patients undergoing proton therapy for prostate cancer were enrolled in an institutional review board-approved imaging study. Each participant underwent 2 CBCT scans on a gantry-mounted proton therapy CBCT system: (1) standard CBCT with software-based scatter correction and (2) CBCT with the 2D ASG. Acquisition parameters and imaging dose were identical. Quantitative image quality metrics, including CT number error and artifact amplitude relative to planning CT, adipose-soft tissue contrast, contrast-to-noise ratio, peak signal-to-noise ratio, and structural similarity index measure, were calculated. Differences between configurations were assessed using the Wilcoxon signed-rank test.

RESULTS

Compared with scatter-corrected clinical CBCT, the 2D ASG reduced median (min-max) soft tissue CT number errors from 169 (150-185) HU to 38 (26-54) HU (P = .002), indicating reduced scatter-induced artifacts. Median soft tissue contrast increased from 62 (42-74) HU to 125 (97-144) HU (P = .002), and contrast-to-noise ratio improved from 0.99 (0.37-1.38) to 1.57 (1.28-1.75) (P = .002). Structural similarity index measure relative to the planning CT increased from 0.53 (0.38-0.67) to 0.87 (0.85-0.93) (P = .002). Similar improvements were observed for peak signal-to-noise ratio and artifact amplitude.

CONCLUSIONS

This first prospective evaluation of a prototype 2D ASG-equipped CBCT system for proton therapy demonstrated significant improvements in CT number accuracy and soft tissue visualization, supporting its potential role in adaptive proton therapy workflows.