[Paper Review] A Comparison of Proton Stopping Power Measured with Proton CT and x-ray CT in Fresh Post-Mortem Porcine Structures

Don F. DeJongh, Ethan A. DeJongh+14 more|arXiv (Cornell University)|Dec 11, 2020

Radiation Therapy and Dosimetry33 references30 citations

TL;DR

This study directly compares proton stopping power (RSP) measured via proton CT (pCT) with that derived from x-ray CT in fresh post-mortem porcine head and thoracic tissues. Using a prototype clinical pCT system with single proton tracking, the authors found RSP agreement within 1-2% for soft tissues and up to 7% discrepancy for compact bone, with up to 40% error in complex cavitated regions like sinuses. The results demonstrate pCT's potential to reduce range uncertainties in proton therapy, enabling lower-dose treatment planning with tighter margins.

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.

Motivation & Objective

• To evaluate the accuracy of proton CT (pCT) in measuring proton stopping power (RSP) relative to x-ray CT in clinically relevant biological tissues.
• To assess the potential of pCT to reduce range uncertainties in proton therapy by directly measuring RSP, thereby enabling smaller treatment margins.
• To investigate discrepancies in RSP measurements between pCT and x-ray CT in complex anatomical regions such as sinuses and tympanic bullae.
• To demonstrate the feasibility of using pCT for low-dose, high-accuracy treatment planning in a clinically realistic setting.
• To explore the utility of proton radiography (pRad) for daily range verification by comparing pRad with digitally reconstructed radiographs (DRRs)

Proposed method

• Acquired proton CT (pCT) images using a prototype clinical proton imaging system with single proton tracking at the Northwestern Medicine Chicago Proton Center.
• Measured individual proton trajectories and residual ranges using tracking detectors and a multi-stage range detector to reconstruct 3D RSP maps.
• Acquired x-ray CT scans of the same porcine samples immediately before or after pCT scans for direct comparison.
• Used beam delivery with multiple proton energies and 90 angular projections (4° increments) to enable pCT reconstruction.
• Performed offline reconstruction of pCT data using a software platform that processes data in real time during acquisition.
• Generated difference maps between pRad images and digitally reconstructed radiographs (DRRs) from x-ray CT to assess range verification potential.

Experimental results

Research questions

• RQ1 How does proton stopping power (RSP) measured by pCT compare to that derived from x-ray CT in soft tissues and compact bone?
• RQ2 What are the discrepancies in RSP measurements between pCT and x-ray CT in complex anatomical regions such as sinus cavities and tympanic bullae?
• RQ3 To what extent can pCT reduce range uncertainties in proton therapy compared to current x-ray CT-based planning?
• RQ4 Can proton radiography (pRad) effectively verify proton range in vivo by comparing with DRRs from planning CT?
• RQ5 How do variations in x-ray CT scanner settings (high-dose vs. low-dose) affect RSP consistency in comparison to pCT?

Key findings

• Proton stopping power (RSP) measured by pCT agreed with x-ray CT-derived values within 1% to 2% for soft tissues.
• Discrepancies in RSP reached up to 7% for compact bone when comparing pCT and x-ray CT results.
• The largest discrepancies—up to 40%—were observed in cavitated regions with mixed tissue, air, and bone content, such as sinus cavities and tympanic bullae.
• pRad images showed strong potential for daily range verification by revealing range discrepancies when compared to DRRs from x-ray CT.
• The study demonstrated that pCT can achieve high-accuracy RSP mapping with low radiation dose, supporting its use in low-dose treatment planning.
• The results suggest that pCT could enable significant reduction in treatment margins, particularly in regions with high anatomical complexity.