Position sensitive detectors in proton therapy: online monitoring of the beam position

Abstract Spot-scanning is a highly dynamic treatment method in proton therapy, tailored to each tumor shape individually. By superimposing many single spots all three spatial axes, prescribed dose applied the volume. To minimize inhomogeneity across this volume, tight constraints on beam position accuracy apply: for below 1%, longitudinal of order 1 mm necessary, whereas lateral plane, needs be roughly one magnitude better. Longitudinal control achieved through selecting energy; laterally, by two sweeper magnets, allowing changes within milliseconds. Such dynamics and require an online measurement enable maintain high quality assurance. Gantry 2 at Center Proton Therapy PSI operates plane parallel strip ionization chamber purpose as final beamline element before patient. The foil-based detector design optimized in-situ placement axis keeps disturbance minimum. A pitch allows reconstruct Gaussian profile with desired accuracy. These profiles are analyzed (and verified) during spot-by-spot basis, next spot applied, introducing dead time after every spot. Applying 50, 000 introduces challenge data acquisition — processing terms keeping times reasonable. Data pileup long mitigated front-end electronics early-stage processing, without compromising measured position.