Pre-treatment dosimetric verification by means of a liquid-filled electronic portal imaging device during dynamic delivery of intensity modulated treatment fields

Background and purpose: Although intensity modulated radiation therapy is c haracterized by three-dimensional dose distributions which are often superi or to those obtained with conventional treatment plans, its routine clinica l implementation is partially held back by the complexity of the beam verif ication. This is even more so when a dynamic multileaf collimator (dMLC) is used instead of a segmented beam delivery. We have therefore investigated the possibility of using a commercially available, liquid-filled electronic portal imaging device (EPID) for the pre-treatment quality assurance of dy namically delivered dose distributions. Methods and materials: A special acquisition mode was developed to optimize the image acquisition speed for dosimetry with the liquid-filled EPID. We investigated the accuracy of this mode for 6 and 18 MV photon beams through comparison with film and ion chamber measurements. The impact of leaf spee d and pulse rate fluctuations was quantified by means of dMLC plans especia lly designed for this purpose. Other factors influencing the accuracy of th e dosimetry (e.g. the need for build-up, remanence of the ion concentration in the liquid and bulging of the liquid at non-zero gantry angles) were st udied as well. We finally compared dosimetric EPID images with the correspo nding image prediction delivered without a patient in the beam. Results: The dosimetric accuracy of the measured dose distribution is simil ar to2% with respect to film and ion chamber measurements. The accuracy dec lines when leaf speed is increased beyond 2 cm/s, but is fairly insensitive to accelerator pulse rate fluctuations. The memory effect is found to be o f no clinical relevance. When comparing the acquired and expected distribut ions, an overall agreement of 3% can be obtained, except at areas of steep dose gradients where slight positional shifts are translated into large err ors. Conclusions: Accurate dosimetric images of intensity modulated beam profile s delivered with a dMLC can be obtained with a commercially available, liqu id-filled EPID. The developed acquisition mode is especially suited for fas t and accurate pre-treatment verification of the intensity modulated fields . (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.