* Corresponding author. E-mail address: vladimir.peskov@cern.ch (V. Peskov). Nuclear Instruments and Methods in Physics Research A 454 (2000) 136}141 Evaluation of a GEM and CAT-based detector for radiation therapy beam monitoring A. Brahme, M. Danielsson, C. Iacobaeus, J. Ostling, V. Peskov*, M. Wallmark Department of Medical Radiation Physics, Karolinska Institute, Stockholm, Sweden Department of Physics in Frescati, Royal Institute of Technology, 24 Frescativagen, 10405 Stockholm, Sweden Abstract We are developing a radiation therapy beam monitor for the Karolinska Institute. This monitor will consist of two consecutive detectors con"ned in one gas chamber: a `keV-photon detectora, which will allow diagnostic quality visualization of the patient, and a `MeV-photon detectora, that will measure the absolute intensity of the therapy beam and its position with respect to the patient. Both detectors are based on highly radiation resistant gas and solid photon to electron converters, combined with GEMs and a CAT as ampli"cation structures. We have performed systematic studies of the high-rate characteristics of the GEM and the CAT, as well as tested the electron transfer through these electron multipliers and various types of converters. The tests show that the GEM and the CAT satisfy all requirements for the beam monitoring system. As a result of these studies we successfully developed and tested a full section of the beam monitor equipped with a MeV-photon converter placed between the GEM and the CAT.  2000 Elsevier Science B.V. All rights reserved. 1. Introduction Radiation therapy, today is applied to approxim- ately 50% of all cancer patients with noticeable results: &45% of the treated patients have up to a 5-year survival time [1]. The success of the method is based on the fact that in a very narrow dose range, the tumor could be e!ectively treated without fatal damage to healthy tissues. The very promising three-dimensional intensity-modulated radiation treatment is under development today which will allow a further increase of the treatment outcome [2]. During the treatment it is extremely important to precisely monitor the absolute intensity of the beam and its position with respect to the tumor and organs at risk. Unfortunately, there is a large gap between com- mercially available beam monitors (portal imaging devices) and real clinical needs. This stimulates intense e!orts to develop improved monitors that will provide fast, on line, information about the beam parameters and the patient alignment. Some of these monitors, for example liquid ionization chambers [1] or amorphous silicon detectors [3], are very close to meet most of the requirements. However, these devices are still unique today. We are planning to develop a rather simple, low cost, rigid and keV-photon-sensitive portal imag- ing device for advanced beam monitoring at the Karolinska Hospital (Stockholm). It will be based on gas and solid secondary-electron converters of radiation combined with planar gaseous 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 8 1 5 - 9