Gas Electron Multipliers - GEM

A new Gas Electron Multiplier (GEM) detector was installed on a clinical linear accelerator (retro-fit), and integrated in the software open-radART. In several test plans, images from the GEM detector and the aSi:H flat panel devices were compared.

Fig.1: Schematic illustration of the detector geometry (not a scale). High energy photons interact in the converter plates and create primary electrons. Those electrons ionize the gas and the secondary electrons are drifted through the chamber down to the readout plate by the applied electric field. When passing a GEM the electrons are multiplied in order to compensate for losses in the converters. Low energy photons will mainly interact at the top of the detector, either in the combined drift electrode/low-energy-converter plate or in the gas volume of the drift region. 

Fig.2: GEMini ED detector installation on a clinical linear accelerator at the Universitätsklinik für Radiotherapie und Radio-Onkologie in Salzburg, Austria. 


Fig.3: Comparison of very first LasVegas phantom images from aSi:H flat panel detector (left) and GEM detector (right). The aSi:H detector image is fully multilevel gain and bad pixel corrected, while the preliminary image acquired with the GEM detector is only 2-point-gain-offset corrected without a bad pixel interpolation.

Relating to the very first results, the GEM detector has proven its potential to be a suitable imaging device in radiotherapy. However, further investigations need to be done to gain more expertise on tuning of the new detector’s properties and to quantify aging effects due to radiation damages.

Further studies of radiation hardness, stability of calibration and temperature behavior can soon be performed on the linac, since the mechanical integration in the gantry arm was performed quite smoothly. Now, it is easy to switch between the amorphous silicon panel, which will still be used clinically, and a research operation mode, to investigate that new device in details. Technically, the installation allows clinical imaging with the GEMini system.


Key Personnel:

  • DI Mitterlechner B., Institute for Research and Development on Advanced RadiationTechnologies (radART), Paracelsus Medical University, Salzburg, Austria
  • Mag. Deutschmann H., Institute for Research and Development on Advanced RadiationTechnologies (radART), Paracelsus Medical University, Salzburg, Austria



  • A novel radiotherapy imaging device: GEM versus amorphous silicon panels., Mitterlechner B.2, Huber S.1, Mooslechner M.2, Sedlmayer F.1,2, Deutschmann H.1,2 (11th Biennal ESTRO, London 2011 May)

Oral Presentations:

  • Erste klinische Implementierung eines Gas Electron Multipliers (GEM) Detektor an einem Linearbeschleuniger, Huber S.1,  Mitterlechner B.2, Mooslechner M.2, Sedlmayer F.1,2, Deutschmann H.1,2 (Drei-Länder-Tagung ÖGMP, DGMP, SGSMP, Wien 2011)


1 Institute for Research and Development on Advanced RadiationTechnologies (radART), Paracelsus Medical University, Salzburg, Austria

2 Department of Radiotherapy and Radio-Oncology at the Salzburger Landeskliniken, Salzburg, Austria