Deep characterization of a fast monitoring system for radiotherapeutic proton beams based on scintillating screens and a CCD camera

On August 2004, in the issue of IEEE Transaction on Nuclear Science Vol. 51, n. 4 was published a manuscript describing extensively a fast and accurate quality control system for radiotherapeutic proton beams. The system was developed for a facility, active since March 2002 at Laboratori Nazionali del Sud (Catania, Italy), in which 62 MeV proton beams are employed for treatment of ocular diseases. It consists of a scintillating screen mounted perpendicularly to the beam axis at a fixed distance and observed by a highly sensitive CCD (charge-coupled device) camera. In the manuscript were reported preliminary experimental results and the comparison of the system performances with respect to other conventional systems. In this work is presented a deeper physics characterization of the system in terms of system yield versus proton beam dose rate and its linearity, contrast and background correction, signal to noise ratio and spatial resolution measured as MTF values. The latter was measured using both a sinusoidal pattern method and an edge pattern method. The equivalence between these two methods of MTF evaluation is demonstrated for proton beams. Finally, MTF values calculated for the presented system by means of the edge pattern method is compared with routine dosimetric system values together with associated spatial resolutions.