LAUSR

Proton computed tomography (pCT) has recently seen considerable research interest as a means of reducing range uncertainties in proton therapy, by reconstructing directly relative stopping power to water (RSP). Recent detector developments have permitted the development of two list-mode pCT scanner prototypes based on broad (passively scattered) proton beam delivery [1] , [2] . In this Monte Carlo (MC) simulation study, the application of fluence field modulated computed tomography (FFMCT), initially developed for X-ray CT [3] , to pCT is presented. Fluence modulated pCT (FMpCT) scans can be acquired by utilizing pencil beam (PB) scanning and can achieve variable image quality in a pCT image. Dose calculation accuracy may thus be simultaneously preserved in the beam path in parallel with imaging dose reduction. Using MC simulations of an ideal list-mode pCT scanner, a uniform cylinder and two patients were studied. Regions of interests (ROI) were selected for high image quality and only PBs intercepting them preserved full fluence (FF). Image quality was investigated in terms of accuracy and noise of the reconstructed RSP. Dose calculation accuracy on FMpCT images was evaluated in terms of dose volume histograms (DVH), range difference (RD) for beam-eye-view (BEV) dose profiles and gamma evaluation. Furthermore, the concept of FMpCT was tested on experimental data by creating pseudo FMpCT scans from broad beam scans acquired with a pCT prototype. For the virtual phantoms, FMpCT noise in ROIs at 1% of FF was equivalent to that of FF images. Integral imaging dose reduction up to 56% was achieved for the two patients for that modulation. Corresponding proton dose calculations on FMpCT images agreed to those from reference images. Applying FMpCT to preliminary experimental data showed that low noise levels and accuracy could be preserved in a ROI.