QUALITY ASSURANCE FOR DYNAMIC MULTILEAF COLLIMATOR MODULATED FIELDS USING A FAST BEAM IMAGING-SYSTEM

A quality assurance procedure was developed for x-ray beam intensity m odulated conformal radiotherapy (IMCRT) using dynamic multileaf collim ators (MLC). The procedure verifies a prescribed intensity modulated x -ray beam pattern in the beam eye's view (BEV) before the treatment pr ocedure is applied to a patient. It Verifies that (a) the leaf sequenc ing computer files were transferred correctly to the linac control com puter; (b) the treatment can be correctly executed without machine fau lts. A fast beam imaging system (BIS) consisting of a Gd2O2S scintilla tion screen, a charge-coupled device (CCD) camera, and a portable pers onal computer (Wellhofer Dosimetrie, Schwarzenbruck, Germany) was comm issioned for this purpose. Measurements for the BIS performance are pr esented in this work. Reference images were derived from MLC leaf sequ encing files that were used to drive a dynamic MLC system (Varian Onco logy Systems, Pale Alto, CA). A correlation method was developed to co mpare the BIS measurements with the calculated reference images. A cor relation coefficient calculated using 26 correct intensity modulated f ields was shown to be a reliable threshold to identify inaccurate trea tment delivery files. The study has demonstrated the feasibility of us ing the BIS and the correlation method to carry out on-line quality as surance tasks for IMCRT treatment fields in the BEV. (C) 1997 American Association of Physicists in Medicine.