A novel portal imaging device for advanced radiation therapy

At Karolinska Institutet in Stockholm, Sweden, a new detector for portal im aging is under development, which could greatly improve the alignment of th e radiation beam with respect to the tumor during radiation treatment. The detector is based on gaseous and solid converters combined with gas electro n multipliers (GEMS) as an amplification structure. The detector will have a large area and will be operated in a very high-rate environment. A protot ype has been built and extensively tested, both with high-rate X-rays as we ll, in a clinical environment at the Karolinska Hospital. High rates and al pha particles could cause discharges in the GEM and discharge propagation f rom GEM to GEM and to the readout electronics. Since reliability is one of the main requirements for the portal imaging device, we performed systemati c studies to find a safe operating range of the device, free from typical h igh-rate problems, such as discharges.