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 modulated conformal radiotherapy (IMCRT) using dynamic multileaf collimators (MLC). The procedure verifies a prescribed intensity modulated x-ray beam pattern in the beam eye's view (BEV) before the treatment procedure is applied to a patient. It verifies that (a) the leaf sequencing computer files were transferred correctly to the linac control computer; (b) the treatment can be correctly executed without machine faults. A fast beam imaging system (BIS) consisting of a Gd2O2S scintillation screen, a charge-coupled device (CCD) camera, and a portable personal computer (Wellhöfer Dosimetrie, Schwarzenbruck, Germany) was commissioned for this purpose. Measurements for the BIS performance are presented in this work. Reference images were derived from MLC leaf sequencing files that were used to drive a dynamic MLC system (Varian Oncology Systems, Palo Alto, CA). A correlation method was developed to compare the BIS measurements with the calculated reference images. A correlation coefficient calculated using 26 correct intensity modulated fields was shown to be a reliable threshold to identify inaccurate treatment delivery files. The study has demonstrated the feasibility of using the BIS and the correlation method to carry out on-line quality assurance tasks for IMCRT treatment fields in the BEV.