Joint Dose Minimization and Variance Optimization for Fluence-Modulated Proton CT

We present an optimization algorithm for fluence-modulated proton computed tomography that allows prescribing spatially inhomogeneous dose and image noise distributions. This is particularly meaningful if proton CT images are used for particle therapy treatment planning and online adaptation, where only the region-or-interest (ROI) around the treatment beam path (i.e. the ROI) is relevant and imaging dose can be reduced elsewhere. This may allow for daily imaging at the treatment site with imaging doses that would not compromise the low dose to healthy tissue made possible by particle therapy. We investigate a typical treatment scenario with two beams and optimize dynamic fluence maps resulting in a dose reduction of 30% outside of the ROI. Increasing the magnitude of dose reduction inside a small volume around an organ-at-risk (OAR) brings the OAR dose to 62% below a scan without fluence modulation. This flexible optimization method may facilitate low-dose image guidance with proton CT.