  Verification of patient specific proton stopping powers obtained in the
patient treatment position can be used to reduce the distal margins needed in
particle beam planning. Proton radiography can be used as a pre-treatment
instrument to verify integrated stopping power consistency with the treatment
planning CT. Although a proton radiograph is a pixel by pixel representation of
integrated stopping powers, the image may also be of high enough quality and
contrast to be used for patient alignment. This investigation qualifies the
accuracy and image quality of a prototype proton radiography system on a
clinical proton delivery system. We have developed a clinical prototype proton
radiography system designed for integration into efficient clinical workflows.
We tested the images obtained by this system for water-equivalent thickness
(WET) accuracy, image noise, and spatial resolution. We evaluated the WET
accuracy by comparing the average WET and rms error in several regions of
interest (ROI) on a proton radiograph of a custom peg phantom. We measured the
spatial resolution on a CATPHAN Line Pair phantom and a custom edge phantom by
measuring the 10% value of the modulation transfer function (MTF). In addition,
we tested the ability to detect proton range errors due to anatomical changes
in a patient with a customized CIRS pediatric head phantom and inserts of
varying WET placed in the posterior fossae of the brain. We took proton
radiographs of the phantom with each insert in place and created difference
maps between the resulting images. Integrated proton range was measured from an
ROI in the difference maps.
