Program Information
Validation Study of Proton Radiography Against CT Data for Quantitative Imaging of Anatomical Changes in Head and Neck Patients
A Hammi*, D Weber , A Lomax , Paul Scherrer Institut, Villigen Psi, Villigen PSI
Presentations
SU-F-J-201 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: In clinical pencil-beam-scanned (PBS) proton therapy,the advantage of the characteristic sharp dose fall-off after the Bragg Peak (BP) becomes a disadvantage if the BP positions of a plan’s constituent pencil beams are shifted,eg.due to anatomical changes. Thus,for fractionated PBS proton therapy, accurate knowledge of the water equivalent path length (WEPL) of the traversed anatomy is critical. In this work we investigate the feasibility of using 2D proton range maps (proton radiography,PR) with the active-scanning gantry at PSI.
Methods: We simulated our approach using Monte Carlo methods (MC) to simulate proton beam interactions in patients using clinical imaging data. We selected six head and neck cases having significant anatomical changes detected in per-treatment CTs.
PRs (two at 0°/90°) were generated from MC simulations of low-dose pencil beams at 230MeV. Each beam’s residual depth-dose was propagated through the patient geometry (from CT) and detected on exiting the patient anatomy in an ideal depth-resolved detector (eg.range telescope). Firstly,to validate the technique, proton radiographs were compared to the ground truth, which was the WEPL from ray-tracing in the patient CT at the pencil beam location. Secondly, WEPL difference maps (per-treatment – planning imaging timepoints) were then generated to locate the anatomical changes, both in the CT (ground truth) and in the PRs. Binomial classification was performed to evaluate the efficacy of the technique relative to CT.
Results: Over the projections simulated over all six patients, 70%, 79% and 95% of the grid points agreed with the ground truth proton range to within ±0.5%, ±1%, and ±3% respectively. The sensitivity,specificity,precision and accuracy were high (mean±1σ, 83±8%, 87±13%, 95±10%, 83±7% respectively).
Conclusion: We show that proton-based radiographic images can accurately monitor patient positioning and in vivo range verification, while providing equivalent WEPL information to a CT scan,with the advantage of a much lower imaging dose.
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