Development of a GEM-based Imaging Detector for Small Field Dosimetry for Proton Therapy Beams

In order to fully utilize the advantages of proton therapy, the beam range, the beam alignment with the tumor and the real-time dose distribution must be accurately known. Small proton fields (with characteristic sizes of less than 3 cm) are often used in radiosurgery, ophthalmic treatments, and as patch fields to augment dose distributions. Accurate planning and quality assurance of such fields are challenging. Gas electron multiplier (GEM)-based dose imaging detectors are capable of providing improved position resolution, dose rate linearity, fast response and accurate reproduction of depth-dose distributions. The purpose of this project is to develop a double-GEM dose imaging detector with the optical readout of scintillation light using a CCD camera, intended for small field measurements. The detector was tested in a 205 MeV proton beam at the Indiana University Cyclotron, during the Indiana University Physics 2012 REU funded by the NSF. It demonstrated linearity in dose rate up to 75 Gy/min. Lateral profiles measured with the GEM detector and radiochromic film agree within 0.4 mm (one pixel size) at 50{\%} isodose. After initial start-up, the detector response was stable within $\pm $5{\%} over a 40 hour time period.