PORTAL DOSIMETRY USING A LIQUID ION-CHAMBER MATRIX - DOSE-RESPONSE STUDIES

Current intensive investigations of electronic portal imaging devices (EPIDs) have prompted their potential application to portal dosimetry. In this paper, the progress made in using a commercial liquid ion cha mber matrix EPID for portal dosimetry is discussed. The pixel value of the liquid ion chamber element was calibrated against dose by exposin g the imager to 6-MV x-ray beams of various intensities obtained with various thicknesses of lead attenuators and a range of source to detec tor distances. Absolute dose values were determined using an ion chamb er on the central axis at the depth of maximum dose in a solid water p hantom. The pixel values of the matrix were determined for various fie ld sizes in order to evaluate the dependence of pixel value on dose at those field sizes. It was confirmed that the pixel value was proporti onal to the square root of the dose rate and was nearly independent of the field size. The 2D pixel values were converted to 2D dose maps in the water phantom after applying a correction for the effect of horns in the flood calibration field. The flood calibration field was used to obtain the relative sensitivity of each pixel. Good agreement was o bserved (normally better than 1% in relative standard deviation) betwe en the converted dose distribution obtained from the pixel matrix and the direct dose measurement using an ion chamber scanned in a water ph antom in regions of shallow dose gradient. For application to on-line portal dosimetry, both the short- and long-term stability of this EPID system were found to be within 1% relative standard deviation. This s ystem, together with an accurate portal dose calculation procedure, ca n be used for on-line radiotherapy dose verification.