A System for Proton Beam Control during Radiotherapy

A system for the on-line control of proton beam profiles and track has been designed and constructed to guarantee the quality assurance of radiotherapy carried out in the Medical-Technical Complex of the Joint Institute for Nuclear Research. To measure the horizontal and vertical profiles of the beam in the treatment room, a multiwire ionization chamber has been designed and constructed. The chamber consists of two anode and three cathode electrodes. Each anode electrode contains 30 wires 0.1 mm in diameter separated by 3 mm. To control the beam track, four industrial semiconductor diodes 2D212A designed for radio engineering application were used. The system is installed in front of the first collimator in the peripheral part of the beam, so that it does not disturb the operated part of the beam. Output signals from the ionization chamber and the diodes are processed by a specially constructed electronic device connected to a personal computer. It utilizes 64-channel 16-bit charge-current converters (TERA chip). One-year's experience of the operation of the system in proton therapy treatment sessions has shown its high reliability and sensitivity to proton beam parameters. The accuracy of controlling the asymmetry of the beam profiles is 2% and the track deviations are equivalent to 0.2 mm of water.