Proton Computed Tomography Imaging for Proton Radiation Therapy

CONCLUSION Proton CT has the potential to substantially improve the range accuracy of proton beams and to provide a low-dose imaging modality for daily image guidance. Careful evaluation of this novel technique is underway. BACKGROUND Proton CT is a novel tomographic imaging modality, which has become a realistic possibility with the increasing availability of rotating proton gantries and sufficiently high proton energies to penetrate the patient. Proton CT is hypothesized to improve the accuracy of proton treatment planning and to provide a low-dose method for daily image-guidance in proton therapy. In recent years, we have constructed a prototype proton CT scanner and installed it on a proton research beam line for further study . The scanner detects individual protons up to a maximum rate of about 105 protons/sec and records their entry and exit coordinates and directions with silicon strip detectors, as well as their residual energy by stopping them in an augmented 18-crystal CsI calorimeter. After calibrating the calorimeter response to water equivalent path length (WEPL), the scanner measures the integral of the relative stopping power (RSP) along the path of each proton The distribution of RSP values can then be reconstructed based on the WEPL values of many protons intersecting the object from multiple directions using advanced iterative algorithms based on projection onto hyperplanes. EVALUATION We have carefully calibrated the scanner with polystyrene plates of known thickness and RSP and obtained first images of a spherical quality assurance phantom (Lucy, Standard Imaging). We are now performing rigorous testing with CT QA phantoms (Catphan, The Phantom Laboratory) to compare characteristics including dose-fluence-noise relationship, sensitometry, spatial resolution, and low-contrast resolution with a state-of-the-art X-ray CT scanner. DISCUSSION While minimization of current proton range errors in the presence of tissue inhomogeneities is important for proton treatment planning, acceptable image quality at low doses is important for image guidance and target delineation, which will be evaluated within this project.