A METHOD AND CONTRAST-DETAIL PHANTOM FOR THE QUANTITATIVE ASSESSMENT OF RADIOTHERAPY PORTAL IMAGING-SYSTEMS

Several designs of electronic portal imaging systems for radiotherapy have been reported commercially available, as well as the familiar rad iographic screen-film combinations. In order to evaluate imaging perfo rmance of these, and future, systems a prototype contrast-detail test object has been developed in conjunction with a method for determining contrast from a model of the X-ray spectra produced by linear acceler ators. Several existing test objects rely on qualitative or semi-quant itative estimates of contrast. In this technique, quantitative estimat es of contrast may be determined for each detail size from realistic e stimates of the X-ray spectrum by using a spectral model either in con junction with narrow-beam attenuation measurements on the linear accel erator used, or by using the nominal value of maximum photon energy. T his technique should facilitate the comparison of imaging systems used with linear accelerators of different energies as well as providing a quantitative quality control tool for regular measurements of imaging performance. Examples of the use of the test object in the evaluation of several commercial screen-film combinations are given.