Pre-treatment patient-specific stopping power by combining list-mode proton radiography and x-ray CT

• Published in: Physics in medicine & biology Vol. 62; no. 17; pp. 6836 - 6852
• Main Authors: Collins-Fekete, Charles-Antoine, Brousmiche, Sébastien, Hansen, David C, Beaulieu, Luc, Seco, Joao
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 03.08.2017
• Subjects: Algorithms; Calibration; Geant4 Monte Carlo; Head - diagnostic imaging; Humans; list-mode proton radiography; Lung - diagnostic imaging; Pelvis - diagnostic imaging; Phantoms, Imaging; proton imaging; proton stopping power; proton therapy; Protons; Radiotherapy Planning, Computer-Assisted - methods; range uncertainty; Tomography, X-Ray Computed - methods; x-ray CT; X-Rays
• ISSN: 0031-9155; 1361-6560; 1361-6560
• DOI: 10.1088/1361-6560/aa7c42

The relative stopping power (RSP) uncertainty is the largest contributor to the range uncertainty in proton therapy. The purpose of this work was to develop a systematic method that yields accurate and patient-specific RSPs by combining (1) pre-treatment x-ray CT and (2) daily proton radiography of the patient. The method was formulated as a penalized least squares optimization problem (argmin(∥Ax−b∥22)). The parameter A represents the cumulative path-length crossed by the proton in each material, separated by thresholding on the HU. The material RSPs (water equivalent thickness/physical thickness) are denoted by x. The parameter b is the list-mode proton radiography produced using Geant4 simulations. The problem was solved using a non-negative linear-solver with x⩾0. A was computed by superposing proton trajectories calculated with a cubic or linear spline approach to the CT. The material's RSP assigned in Geant4 were used for reference while the clinical HU-RSP calibration curve was used for comparison. The Gammex RMI-467 phantom was first investigated. The standard deviation between the estimated material RSP and the calculated RSP is 0.45%. The robustness of the techniques was then assessed as a function of the number of projections and initial proton energy. Optimization with two initial projections yields precise RSP ( 1.0%) for 330 MeV protons. 250 MeV protons have shown higher uncertainty ( 2.0%) due to the loss of precision in the path estimate. Anthropomorphic phantoms of the head, pelvis, and lung were subsequently evaluated. Accurate RSP has been obtained for the head (μ=0.21±1.63%), the lung (μ=0.06±0.99%) and the pelvis (μ=0.90±3.87%). The range precision has been optimized using the calibration curves obtained with the algorithm, yielding a mean R80 difference to the reference of 0.11  ±0.09%, 0.28  ±  0.34% and 0.05±0.06% in the same order. The solution's accuracy is limited by the assumed HU/RSP bijection, neglecting inherent degeneracy. The proposed formulation of the problem with prior knowledge x-ray CT demonstrates potential to increase the accuracy of present RSP estimates.