Dose/sensitivity in proton computed tomography

Proton therapy has become an established form of cancer treatment, but dose calculations and treatment planning are routinely performed based on X-ray computed tomography (XRCT), which requires a conversion to proton stopping power. A more appropriate method to directly measure stopping power and dose is proton computed tomography (pCT), where high-energy protons are measured after traversing completely through the patient. Proton radiographs and pCT have historically been limited by blurring due to multiple scattering. However, proton-by- proton track reconstruction techniques, measuring entry positions and exit positions and energies of each scanning proton, promise to greatly improve the spatial resolution of proton radiographs. We use simplified physical models of proton transport (including Bethe-Bloch energy loss, energy straggling, and multiple Coulomb scattering) in the 0-300 MeV energy range of interest to analytically quantify the tradeoffs and scaling between dose, spatial resolution, density resolution, and scanning voxel size. Monte Carlo results and comparisons to this scaling are generated with a small "fast" Monte Carlo code specifically written for proton transport and pCT (pint).