Measurement of depth-dose distribution of protons by an imaging plate

By using an imaging plate (IP), we have developed a novel, simple method for depth-dose distribution measurement of clinical proton beams. When protons are obliquely incident upon the IP, the energy is deposited closer to the surface of the photo-stimulated luminescence (PSL) material than in the case of normal incidence. On the other hand, the spatial distribution of deposited energy varies considerably with variations in proton energy and results in a dependence of the IP response on linear-energy transfer (LET). Through a combination of these factors, for the oblique incidence, IP response to protons may be relatively enhanced around the Bragg peak because the observed PSL intensity depends on the position (depth) at which the energy is deposited. To examine this geometrical enhancement, a simple calculation is performed. The effect is thought to reduce the dependence of IP response on LET so that the response of an IP may approach that of a parallel-plate ionization chamber. Experimental results are given for protons of various angles of incidence (θ) on the IP. The proposed method may be useful for conducting quick checks of depth-dose distributions in proton-therapy facilities.