Ion radiography as a tool for patient set-up and image guided particle therapy: a Monte Carlo study

This study investigate the use of ion radiography as a tool for patient set-up and tumor tracking capabilities for image guided particle therapy (IGPT) using Monte Carlo simulations. One pediatric, two lung and one liver cancer patients were considered in this study. For each patient, 230 and 330 MeV proton, and 500 MeV/nucleon carbon ion pencil beams were simulated through their computed tomography (CT) data set using GEANT4.9.0. Energy, position and direction cosines of each particle were recorded in front and behind the patient. Ion radiographs were subsequently reconstructed using a dedicated in-house software. The image quality was assessed by evaluating the contrast-to-noise ratio of the tumor and its surrounding tissue. In the lung and liver cases, each CT phase of the breathing cycle was treated individually and dynamic sequences were later produced to appreciate tumor motion. Reconstructed radiographs show high spatial resolution. This allows for excellent imaging capabilities in pediatric patients, comparable to X-ray imaging at a fraction of the imaging dose. There is clear visualization of the tumor edges in the lung due to the great contrast-to-noise ratio between the tumor and its surrounding tissues; tumor motion is observed and comparable to 4D CT data thus allowing for on-line tumor tracking during ion radiotherapy. Conversely, tumor edge detection is difficult in liver, and fiducial markers are required to attempt indirect tumor tracking for IGPT. Ion radiographs with high spatial resolution can be generated using the PR-creator software resulting in pediatric patient set-up capabilities at a fraction of the current imaging dose, as well as the capacity to track moving targets in order to achieve IGPT.