Besides high proton stopping power accuracy, proton computed tomography (pCT) offers the prospect of frequent image guidance at the isocenter, enabling both image guided and adaptive proton therapy. The modality is particularly appealing due to reportedly better contrast to noise ratio at equivalent dose when compared to x-ray computed tomography (CT) imaging. The concept of fluence modulated proton computed tomography (FMpCT), adopting the ideas of fluence field modulation from x-ray CT, may allow a further reduction of the imaging dose. This can be achieved by modulating the proton fluence within a projection, allowing the eventual prescription of image quality in relevant portions of the image, such as the proton beam path. Fluence modulation within a projection is readily achievable by using the pencil beam scanning (PBS) capability of modern proton therapy beamlines. Image quality prescription can be performed either in a “forward planning” fashion, where high fluence is preserved for pencil beams intersecting a region of interest (ROI), or in an “inverse planning” fashion where a model linking image quality and proton pencil beam (PB) fluence is required for optimization.

In this contribution we will present original proof of principle simulation studies supporting the feasibility of FMpCT for simple phantoms and clinically relevant patient cases. Subsequently, results and challenges from the first experimental FMpCT scans on the basis of PBS will be discussed. Finally, the topic of noise reconstruction in pCT will be covered, using simulated pCT scans as a starting point.