A Comparison of Proton Stopping Power Measured with Proton CT and x-ray\n CT in Fresh Post-Mortem Porcine Structures

Abstract

Purpose: Currently, calculations of proton range in proton therapy patients\nare based on a conversion of CT Hounsfield Units of patient tissues into proton\nrelative stopping power. Uncertainties in this conversion necessitate larger\nproximal and distal planned target volume margins. Proton CT can potentially\nreduce these uncertainties by directly measuring proton stopping power. We aim\nto demonstrate proton CT imaging with complex porcine samples, to analyze in\ndetail three-dimensional regions of interest, and to compare proton stopping\npowers directly measured by proton CT to those determined from x-ray CT scans.\n Methods: We have used a prototype proton imaging system with single proton\ntracking to acquire proton radiography and proton CT images of a sample of\nporcine pectoral girdle and ribs, and a pig's head. We also acquired close in\ntime x-ray CT scans of the same samples, and compared proton stopping power\nmeasurements from the two modalities. In the case of the pig's head, we\nobtained x-ray CT scans from two different scanners, and compared results from\nhigh-dose and low-dose settings.\n Results: Comparing our reconstructed proton CT images with images derived\nfrom x-ray CT scans, we find agreement within 1% to 2% for soft tissues, and\ndiscrepancies of up to 6% for compact bone. We also observed large\ndiscrepancies, up to 40%, for cavitated regions with mixed content of air, soft\ntissue, and bone, such as sinus cavities or tympanic bullae.\n Conclusions: Our images and findings from a clinically realistic proton CT\nscanner demonstrate the potential for proton CT to be used for low-dose\ntreatment planning with reduced margins.\n