  Currently, calculations of proton range in proton therapy patients are based
on a conversion of CT Hounsfield Units of patient tissues to 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 have used a
prototype proton imaging system to acquire proton radiography and proton CT
images of a sample of pork shoulder and ribs, as well as of a pig's head. Close
in time, we also acquired X-Ray CT scans, and compared proton stopping power
measurements from the two modalities. We find agreement within 1\% to 2\% for
soft tissues, and discrepancies of up to 6\% for compact bone. We observe the
largest discrepancies, up to 30\%, for cavitated regions with mixed content of
air, soft tissue, and bone, such as sinus cavities or tympanic bullae. If these
results apply in general, it may be possible to substantially reduce
uncertainty margins for many treatment plans, while avoiding regions with
higher uncertainties. Our comparison of proton radiography data with a
digitally reconstructed radiograph from the X-Ray CT demonstrates the potential
of this modality for daily pre-treatment range verification.
