Dosimetric verification of intensity modulated beams produced with dynamicmultileaf collimation using an electronic portal imaging device

Dose distributions can often be significantly improved by modulating the tw o-dimensional intensity profile of the individual x-ray beams. One techniqu e for delivering intensity modulated beams is dynamic multileaf collimation (DMLC). However, DMLC is complex and requires extensive quality assurance. In this paper a new method is presented for a pretreatment dosimetric veri fication of these intensity modulated beams utilizing a charge-coupled devi ce camera based fluoroscopic electronic portal imaging device (EPID). In th e absence of the patient, EPID images are acquired for all beams produced w ith DMLC. These images are then converted into two-dimensional dose distrib utions and compared with the calculated dose distributions. The calculation s are performed with a pencil beam algorithm as implemented in a commercial ly available treatment planning system using the same absolute beam fluence profiles as used for calculation of the patient dose distribution. The met hod allows an overall verification of (i) the leaf trajectory calculation ( including the models to incorporate collimator scatter and leaf transmissio n), (ii) the correct transfer of the leaf sequencing file to the treatment machine, and (iii) the mechanical and dosimetrical performance of the treat ment unit. The method was tested for intensity modulated 10 and 25 MV photo n beams; both model cases and real clinical cases were studied. Dose profil es measured with the EPID were also compared with ionization chamber measur ements. In all cases both predictions and EPID measurements and EPID and io nization chamber measurements agreed within 2% (1 sigma). The study has dem onstrated that the proposed method allows fast and accurate pretreatment ve rification of DMLC. (C) 1999 American Association of Physicists in Medicine . [S0093-2405(99)01411-X].