Proton Computed Tomography

This chapter presents the principles of proton computed tomography (pCT) and reviews its clinical applications. The emphasis is on image reconstruction from projected data along proton paths, which may not necessarily be straight lines through the object to be imaged. A successful implementation of pCT would avoid the ambiguities of mapping X-ray computed tomography (xCT) Hounsfield Units (HU, which is related to the X-ray attenuation coefficients) to electron densities and would allow actual dose distribution as well as verification of patient position in the treatment room. The availability of pCT in the treatment room predicts very accurately the position of the Bragg peak within the patient's body, resulting in a maximum dose delivery to the targeted tumor and successful sparing of the surrounding normal tissues. Furthermore, a successful integration of pCT with proton therapy may lead to the ultimate form of image-guided 3D conformal radiation therapy, which has the potential to deliver the optimal dose to any point within the patient and provide arbitrarily shaped inhomogeneous dose distributions as desired. Image formation for pCT, similar to other imaging modalities, relies on the interaction of incident energy with the tissues inside the body. Knowledge of the interaction and the accuracy of measuring the difference of the exit energy from the incident energy determine the quality of the reconstructed image about the body internals. In the important energy range for pCT (1-250 MeV), the mean energy loss of protons per unit path length, also called stopping power dE/dr, is mainly due to the ionizations and atomic excitations and is well described by the Bethe Bloch theory.