Technical Note: A direct ray-tracing method to compute integral depth dose in pencil beam proton radiography with a multilayer ionization chamber Paolo Farace and Roberto Righetto Proton Therapy Unit, Hospital of Trento, Trento 38100, Italy Sylvain Deffet Institute of Information and Communication Technologies, Université Catholique de Louvain (UCL), Louvain-La-Neuve, 1348, Belgium Arturs Meijers a) and Francois Vander Stappen Ion Beam Applications (IBA), Louvain-la-Neuve, 1348, Belgium (Received 19 May 2016; revised 3 October 2016; accepted for publication 16 October 2016; published 9 November 2016) Purpose: To introduce a fast ray-tracing algorithm in pencil proton radiography (PR) with a multi- layer ionization chamber (MLIC) for in vivo range error mapping. Methods: Pencil beam PR was obtained by delivering spots uniformly positioned in a square (45 × 45 mm 2 field-of-view) of 9 × 9 spots capable of crossing the phantoms (210 MeV). The exit beam was collected by a MLIC to sample the integral depth dose (IDD MLIC ). PRs of an electron- density and of a head phantom were acquired by moving the couch to obtain multiple 45 × 45 mm 2 frames. To map the corresponding range errors, the two-dimensional set of IDD MLIC was compared with (i) the integral depth dose computed by the treatment planning system (TPS) by both analytic (IDD TPS ) and Monte Carlo (IDD MC ) algorithms in a volume of water simulating the MLIC at the CT, and (ii) the integral depth dose directly computed by a simple ray-tracing algorithm (IDD direct ) through the same CT data. The exact spatial position of the spot pattern was numerically adjusted testing different in-plane positions and selecting the one that minimized the range differences between IDD direct and IDD MLIC . Results: Range error mapping was feasible by both the TPS and the ray-tracing methods, but very sensitive to even small misalignments. In homogeneous regions, the range errors computed by the direct ray-tracing algorithm matched the results obtained by both the analytic and the Monte Carlo algorithms. In both phantoms, lateral heterogeneities were better modeled by the ray-tracing and the Monte Carlo algorithms than by the analytic TPS computation. Accordingly, when the pencil beam crossed lateral heterogeneities, the range errors mapped by the direct algorithm matched better the Monte Carlo maps than those obtained by the analytic algorithm. Finally, the simplicity of the ray-tracing algorithm allowed to implement a prototype procedure for automated spatial alignment. Conclusions: The ray-tracing algorithm can reliably replace the TPS method in MLIC PR for in vivo range verification and it can be a key component to develop software tools for spatial alignment and correction of CT calibration. C 2016 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4966703] Key words: proton radiography, multi layer ionization chamber, Monte Carlo, range uncertainty, CT calibration 1. INTRODUCTION Despite range accuracy being one of the major challenges of proton therapy, clinical data on range uncertainty are still missing. A systematic collection of measurements is mandatory to allow quantification of actual range uncertainties based on patient data instead of simulations. Many projects are focused on the development of methods for in vivo measurement, 1,2 but most of them are still far from being usable on patients, or from providing absolute range errors. Among the techniques investigated, proton radiography 3 (PR) is in advanced stages of development. A method to obtain PR (with a mean dose <1 cGyE) based on the use of a multilayer ionization chamber (MLIC) has been recently developed and successfully tested on an anthropomorphic phantom. 4 A distinctive advancement in that approach was the integration between PR and the prediction of the corre- sponding integral depth dose (IDD) by the treatment planning system (TPS). Such integration is indispensable to produce a corresponding map of range errors, i.e., to verify the accuracy of CT calibration and of the related stopping power ratio (SPR) assignment. Despite the promising potential, the method would benefit from some improvements. The total time to acquire a whole PR was quite long. The implementation of a universal reference method, which is independent on the use of a specific TPS, 6405 Med. Phys. 43 (12), December 2016 0094-2405/2016/43(12)/6405/8/$30.00 © 2016 Am. Assoc. Phys. Med. 6405