# Medical Applications of the GEMPix
###### Abstract  
The GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region, three Gas Electron Multipliers (GEMs) for signal amplification, and four Timepix ASICs with 55 \(\upmu\)m pixel pitch and a total of 262,144 pixels. A continuous flow of a gas mixture such as Ar:CO\({}_{2}\):CF\({}_{4}\) Ar:CO\({}_{2}\) or propane-based tissue equivalent gas is supplied externally at a rate of 5 L/h. This article reviews the medical applications of the GEMPix. These include relative dose measurements in conventional photon radiation therapy and in carbon ion beams, by which on-line 2D dose images provided a similar or better performance compared to gafchromic films. Depth scans in a water phantom with \({}^{12}\)C ions allowed measuring the 3D energy deposition and reconstructing the Bragg curve of a pencil beam. Microdsometric measurements performed in neutron and photon fields allowed comparing dose spectra with those from Tissue Equivalent Proportional Counters and, additionally, to obtain particle track images. Some preliminary measurements performed to check the capabilities as the detector in proton tomography are also illustrated. The most important on-going developments are: (1) a new, larger area readout to cover the typical maximum field size in radiation therapy of 20 \(\times\) 20 cm\({}^{2}\); (2) a sealed and low-pressure version to facilitate measurements and to increase the equivalent spatial resolution for microdsometry; (3) 3D particle track reconstruction when operating the GEMPix as a Time Projection Chamber. GEMPix; GEMs; Timepix; Medipix; radiation therapy; hadron therapy; microdsometry; gaseous detectors +
Footnote †: journal: Medical Applications of the GEMPix  
place around current and envisaged Quality Assurance (QA) instrumentation for particle therapy. The discussion focussed on:  
1. commissioning of the Treatment Planning System (TPS). This includes experimental data on integral depth dose distributions of monoenergetic pencil beams using the PTW Peakfinder (Physikalisch-Technische Werkstatten Dr. Pychalu GmbH, Freiburg, Germany, [4]), transversal dose profiles in air measured with External Beam Therapy (EBT3) radiochromic films [5], transversal dose profiles in a water phantom with pin-point Ionisation Chamber (IC) like the PTW 31014 [6], uniformity tests of the scanned beam using EBT3 films;
2. determination of absorbed dose to water under reference conditions with Farmer-type IC like the PTW 30013 [7], and calibration of beam monitor chambers used to integrate the dose to patient;
3. determination of procedures and reference values for periodic QA checks. Daily QA checks include spot position accuracy and size checks, and beam energy constancy check, using EBT3 films. Patient-specific pre-treatment QA includes TPS verification plan. The current commissioning and QA procedures at CNAO are described in [8]. The concept of GEM detectors [9; 10], which are mostly used in high-energy physics, was presented, describing the Triple-GEM construction [11] with the standard pad readout made of 128 channels allowing spatial resolution of typically a few mm, the FPGA-based data acquisition system (DAQ) and a specifically designed High Voltage (HV) power supply called HVGEM to supply the required power via seven HV channels. GEMs allow real time track reconstruction, from single particle to high intensity beams. At the same time, the single particle counting pixel detector Timepix [12] was introduced by representatives of the Medipix collaboration [13; 14; 15], explaining its particle tracking capabilities and particle discrimination properties by cluster analysis. The idea to merge these two technologies and combine their advantages to build a gaseous detector with a highly pixelated readout and achieve superior spatial resolution unfolded rapidly, and yielded the design and construction of the first GEMPix prototypes in March 2013. Only a few years later the detector has proven very successful and has been used for several applications.