FAST 2D PHANTOM DOSIMETRY FOR SCANNING PROTON-BEAMS

A quality control system especially designed for dosimetry in scanning proton beams has been designed and tested. The system consists of a s cintillating screen (Gd2O2S:Tb), mounted at the beam-exit side of a ph antom, and observed by a low noise CCD camera with a long integration time. The purpose of the instrument is to make a fast and accurate two -dimensional image of the dose distribution at the screen position in the phantom. The linearity of the signal with the dose, the noise in t he signal, the influence of the ionization density on the signal, and the influence of the field size on the signal have been investigated. The spatial resolution is 1.3 mm (1 s.d.), which is sufficiently small er than typical penumbras in dose distributions, The measured yield de pends linearly on the dose and agrees within 5% with the calculations. In the images a signal to noise ration (signal/l s.d.) of 10(2) has b een found, which is in the same order of magnitude as expected from th e calculations. At locations in the dose distribution possessing a str ong contribution of high ionization densities (i.e., in the Bragg peak ), we found some quenching of the light output, which can be described well by existing models if the beam characteristics are known. For cl inically used beam characteristics such as a Spread Out Bragg peak, th ere is at most 8% deviation from the NACP ionization chamber measureme nts. The conclusion is that this instrument is a useful tool for quick and reliable quality control of proton beams. The long integration-ti me capabilities of the system make it worthwhile to investigate its ap plicability in scanning proton beams and other dynamic treatment modal ities. (C) 1998 American Association of Physicists in Medicine. [S0094 -2405(98)02104-X].