Stopping-Power Ratio Accuracy for Proton Therapy: Photon-Counting Detector Versus Energy-Integrating Detector CT
Abstract
Purpose
Stopping-power ratio (SPR) is used in proton therapy to calculate the radiation dose distributions. SPR depends on tissue composition and can be calculated using multi-energy CT. This study compares the accuracy of SPR calculated using energy-integrating detector CT (EID-CT) and photon-counting detector CT (PCD-CT).
Methods
A phantom (Gammex Multi-Energy CT Phantom, Sun Nuclear) containing nine tissue-like inserts was fitted to represent three different patient sizes (20, 40, and 50 cm lateral width). Scans were acquired on a dual-source dual-energy EID-CT (SOMATOM Force) at kV pairs of 80/150Sn, 90/150Sn, and 100/150Sn kV, and on a PCD-CT (NAEOTOM Alpha) at 120 and 140 kV. Images were reconstructed with a quantitative kernel (Qr40) and 2-mm slice thickness. SPR was calculated inside regions of interest within the inserts using prototype software provided by the manufacturer and compared to ground-truth values based on published literature and public databases assuming a 100 MeV proton energy. Mean absolute percent error (MAPE) was calculated across materials for different phantom sizes and kVs.
Results
Both EID-CT and PCD-CT demonstrated low SPR error relative to ground truth, however, PCD-CT consistently achieved lower error. Across all phantom sizes and materials, the MAPE was 1.12% for PCD-CT at both 120 and 140 kV, indicating no appreciable dependence on tube potential. For EID-CT, SPR accuracy showed a slight dependence on tube potential, with MAPE decreasing from 1.90% (80/Sn150 kV) and 1.78% (90/Sn150 kV) to 1.71% (100/Sn150 kV).
Conclusion
PCD-CT and EID-CT measurements of SPR both agree well with published ground truth, with PCD-CT slightly outperforming EID-CT. The MAPE averaged over all materials and kV setting for was 1.12% and 1.80% for PCD-CT and EID-CT, respectively, suggesting that both scanners yield accurate results for SPR.