BEST IN PHYSICS (MULTI-DISCIPLINARY): A Gantry-Mounted Photon-Counting CT Prototype for On-Board Proton Stopping Power Ratio Estimation
Abstract
Purpose
On-board kV imaging systems on proton therapy machines predominantly rely on scintillator-based energy-integrating detectors (EIDs) for planar imaging and cone-beam CT (CBCT). However, EID-based CBCT is limited by suboptimal image quality and insufficient quantitative accuracy, constraining its utility for online adaptive proton therapy. This study aims to experimentally demonstrate the feasibility and potential advantages of gantry-mounted photon-counting detector (PCD) CT imaging for electron density (ED) and proton stopping power ratio (SPR) estimations with the image object in the exact treatment position.
Methods
A CdTe-based PCD with a 100 μm pixel size and two energy bins was integrated onto the surface of the existing flat-panel EID of an IBA Proteus ONE proton therapy system. PCD-CT scans were acquired using the same protocol as clinical head EID-CBCT. Standardized imaging phantoms with known material compositions were scanned under matched x-ray exposure and beam width conditions to assess non-spectral image quality. Spectral PCD-CT data were further processed to directly compute ED and proton SPRs at 200 MeV, without reliance on empirical HU-to-ED or HU-to-SPR calibration curves.
Results
After matching MTF through reconstruction kernel adjustment, PCD-CT consistently demonstrated higher contrast-to-noise ratio than EID-CBCT across all tested materials, with an average percent improvement of 67% (95% CI: 29%–106%). The on-board PCD-CT achieved a mean absolute percent error of 1.8% for ED estimation and 1.1% for SPR estimation relative to ground-truth values. When plotted against ground truth, PCD-CT–based estimates yielded a linear regression slope of 1.051, a y-intercept of -0.054, and an R2 value of 0.999.
Conclusion
This work demonstrates the feasibility of gantry-mounted PCD-CT imaging on a clinical proton therapy system. Compared with conventional EID-CBCT, PCD-CT provides superior non-spectral image quality and enables accurate on-board estimation of ED and SPR, highlighting its potential to support online adaptive proton therapy and other advanced proton imaging applications.