Technical note: Flat panel proton radiography with a patient specific imaging field for accurate WEPL assessment

Background Proton radiography (PR) uses highly energetic proton beams to create images where energy loss is the main contrast mechanism. Water‐equivalent path length (WEPL) measurements using flat panel PR (FP‐PR) have potential for in vivo range verification. However, an accurate WEPL measurement via FP‐PR requires irradiation with multiple energy layers, imposing high imaging doses. Purpose A FP‐PR method is proposed for accurate WEPL determination based on a patient‐specific imaging field with a reduced number of energies (n) to minimize imaging dose. Methods Patient‐specific FP‐PRs were simulated and measured for a head and neck (HN) phantom. An energy selection algorithm estimated spot‐wise the lowest energy required to cross the anatomy (Emin) using a water‐equivalent thickness map. Starting from Emin, n was restricted to certain values (n = 26, 24, 22, …, 2 for simulations, n = 10 for measurements), resulting in patient‐specific FP‐PRs. A reference FP‐PR with a complete set of energies was compared against patient‐specific FP‐PRs covering the whole anatomy via mean absolute WEPL differences (MAD), to evaluate the impact of the developed algorithm. WEPL accuracy of patient‐specific FP‐PRs was assessed using mean relative WEPL errors (MRE) with respect to measured multi‐layer ionization chamber PRs (MLIC‐PR) in the base of skull, brain, and neck regions. Results MADs ranged from 2.1 mm (n = 26) to 21.0 mm (n = 2) for simulated FP‐PRs, and 7.2 mm for measured FP‐PRs (n = 10). WEPL differences below 1 mm were observed across the whole anatomy, except at the phantom surfaces. Measured patient‐specific FP‐PRs showed good agreement against MLIC‐PRs, with MREs of 1.3 ± 2.0%, −0.1 ± 1.0%, and −0.1 ± 0.4% in the three regions of the phantom. Conclusion A method to obtain accurate WEPL measurements using FP‐PR with a reduced number of energies selected for the individual patient anatomy was established in silico and validated experimentally. Patient‐specific FP‐PRs could provide means of in vivo range verification.