Development and Evaluation of a Pixel Detector System for Pencil Beam Scanning Proton Therapy

At the Paul Scherrer Institute (PSI) in Switzerland, cancer patients are treated with protons. The PSI Gantries work with Pencil Beam Scanning (PBS) technology. For beam profile characterization, a new 2-D planar pixel detector based on Printed Circuit Board technology was developed. The main challenge of the design was the limited amount of 256 readout channels while several thousands of detector signals had to be processed. With PBS, only a few pixels will be hit by the proton beam and give a significant signal, while most of them will not capture any signal. On the detector design, the active area was divided into pixel arrays with equal size, while one array is such big to cover the whole size of the proton beam. By a so-called channel recycling technique, the same pixel number of all pixel arrays were connected together to one readout channel. The proof of concept was verified with a small detector. In the next step, the design was changed such, getting a full-scale detector for Gantry 2. The readout electronics board itself is also a development performed at PSI. A core component of the design is the ADAS1128 chip from ANALOG Devices featuring 128 input channels. The pixel detector is fully integrated into the therapy control system (TCS) through readout electronics with a field-programmable gate array (FPGA) technology. The FPGA technology allows to readout the detector synchronized to the beam delivery. This paper summarizes the features of the pixel detector design, the readout electronics, and shows the first experimental results.