A comparison of proton stopping power measured with proton CT and x-ray CT in fresh postmortem porcine structures

Abstract

Purpose: Currently, calculations of proton range in proton therapy patients are based on a conversion of CT Hounsfield units of patient tissues into proton relative stopping power. Uncertainties in this conversion necessitate larger proximal and distal planned target volume margins. Proton CT can potentially reduce these uncertainties by directly measuring proton stopping power. We aim to demonstrate proton CT imaging with complex porcine samples, to analyze in detail three-dimensional regions of interest, and to compare proton stopping powers directly measured by proton CT to those determined from x-ray CT scans.

Methods: We have used a prototype proton imaging system with single proton tracking to acquire proton radiography and proton CT images of a sample of porcine pectoral girdle and ribs, and a pig's head. We also acquired close in time x-ray CT scans of the same samples and compared proton stopping power measurements from the two modalities. In the case of the pig's head, we obtained x-ray CT scans from two different scanners and compared results from high-dose and low-dose settings.

Results: Comparing our reconstructed proton CT images with images derived from x-ray CT scans, we find agreement within 1% to 2% for soft tissues and discrepancies of up to 6% for compact bone. We also observed large discrepancies, up to 40%, for cavitated regions with mixed content of air, soft tissue, and bone, such as sinus cavities or tympanic bullae.

Conclusions: Our images and findings from a clinically realistic proton CT scanner demonstrate the potential for proton CT to be used for low-dose treatment planning with reduced margins.

Keywords: digitally reconstructed radiograph; iterative algorithm; proton computed tomography; proton imaging; proton radiography; relative stopping power.

Figure 1:

Left: Porcine girdle and ribs, inserted into a blue-wax bucket. Right: Pig’s head immobilized with plastic foam and tape, and with surface-mounted spherical CT markers.

Figure 2:

pCT dose maps for representative 2 mm slices of the pectoral girdle and ribs (top) and pig’s head (bottom) for the case of optimized scan patterns, with corresponding pCT slices on the right.

Figure 3:

Example of a 1 mm thick pCT slice of the custom phantom with inserts of various tissue-equivalent materials with known RSP. Labels are: A: Sinus. B: Dentin. C: Enamel. D: Brain. E: Spinal Cord. F: Spinal Disk. G: Trabecular Bone. H: Cortical Bone.

Figure 4:

Top row: Acquired pRad WET image of the pig’s head, used to create the difference images in the middle and bottom rows. Middle row: Simulated pRad DRR from x-ray CT (Difference from acquired pRad on right). Bottom row: Simulated pRad DRR from pCT (Difference from acquired pRad on right). Noteable differences with the x-ray DRR are visible in the sinus region near coordinates (0,0) and the tympanic bullae near coordinates (8,−2).

Figure 5:

Examples of 1 mm thick CT slices for the porcine pectoral girdle and ribs, showing regions of interest. Top: pCT. Bottom: x-ray CT. Labels are: A: Blue Wax. B: Muscle (Shoulder-Med). C: Muscle (Ribs). D: Air. E: Adipose (Shoulder). F: Compact Bone. G: Trabecular Bone (Shoulder). H: Adipose (Ribs). I: Muscle (Shoulder-Lat)

Figure 6:

Examples of 1 mm thick CT slices for the Pig’s head, showing regions of interest. Images on the top are for pCT, those on the bottom are for low-dose horizontal x-ray CT. Left: sagittal view. Middle: coronal view. Right: axial view. Labels are: A: Adipose. B: Eye. C: Lens. D: Sinus. E: Brain Stem. F: Muscle. G: Brain. H: Skull. I: Mandible. J: Tongue.

Figure 7:

Tympanic bullae contours in one axial CT slice. Left: pCT. Right: Low-dose horizontal x-ray CT.

Figure 8:

Ten-slice averages (a) Pig’s head pCT. (b) Pig’s head vertical x-ray CT. (c) pCT - x-ray CT difference.