ABSTRACT
Electronic portal imaging devices (EPIDs) are mainly used for patient setup verification during treatment but other geometric properties like block shape and leaf positions are also determined. Electronic portal dosimetry allows dosimetric treatment verification. By combining geometric and dosimetric information, the data transfer between treatment planning system (TPS) and linear accelerator can be verified which in particular is important when this transfer is not carried out electronically. We have developed a pretreatment verification procedure of geometric and dosimetric treatment parameters of a 10 MV photon beam using an EPID. Measurements were performed with a CCD camera-based iView EPID, calibrated to convert a greyscale EPID image into a two-dimensional absolute dose distribution. Central field dose calculations, independent of the TPS, are made to predict dose values at a focus-EPID distance of 157.5 cm. In the same EPID image, the presence of a wedge, its direction, and the field size defined by the collimating jaws were determined. The accuracy of the procedure was determined for open and wedged fields for various field sizes. Ionization chamber measurements were performed to determine the accuracy of the dose values measured with the EPID and calculated by the central field dose calculation. The mean difference between ionization chamber and EPID dose at the center of the fields was 0.8 +/- 1.2% (1 s.d.). Deviations larger than 2.5% were found for half fields and fields with a jaw in overtravel. The mean difference between ionization chamber results and the independent dose calculation was -0.21 +/- 0.6% (1 s.d.). For all wedged fields, the presence of the wedge was detected and the mean difference in actual and measured wedge direction was 0 +/- 3 degrees (1 s.d.). The mean field size differences in X and Y directions were 0.1 +/- 0.1 cm and 0.0 +/- 0.1 cm (1 s.d.), respectively. Pretreatment monitor unit verification is possible with high accuracy and also geometric parameters can be verified using the same EPID image.
