THE DOSE-RESPONSE RELATIONSHIP OF A LIQUID-FILLED ELECTRONIC PORTAL IMAGING DEVICE

To use a liquid-filled portal imaging device (EPID) for transmission d osimetry, it is necessary to understand its dosimetric properties. The refore, the relation between the pixel values (i.e., ionization curren ts) of an electronic portal imaging device and the dose rate measured with an ionization chamber in a mini-phantom was investigated. First, a model was introduced to describe the ionization current of the matri x of liquid-filled ionization chambers for pulsed radiation. With this model the relation between ionization current and dose rate is explai ned qualitatively. Next, buildup measurements were performed at differ ent photon beam energies to assess the amount of buildup material requ ired to obtain electronic equilibrium in the detector. This additional buildup material significantly decreased the image quality, which can hamper patient setup verification, at only the 25 MV beam. Pixel valu es were then compared with measurements made with a Farmer-type ioniza tion chamber in a mini-phantom at various dose rates. In addition, the influence of a number of accelerator and EPID settings (photon beam e nergy, pulse rate frequency, gantry rotation angle, and image acquisit ion modes) on the pixel value was investigated. Subsequently, the dose response relationships of three commercially obtained EPIDs of the sa me type were compared. For all types of measurements the relation betw een ionization current and dose rate is described within 1% (1 SD) by an equation with two terms: one term proportional to the square root o f the dose rate and another term linear to the dose rate. For images o btained under a typical clinical situation (applying the ''normal'' ac quisition mode at an 8 MV beam with a pulse rate frequency of 400 Hz a t a transmission dose rate of 100 cGy/min) the contribution of the squ are root and linear term to the EPID signal is 94% and 6%, respectivel y. The weight factors of both terms depend on the photon beam energy, pulse rate frequency, and image acquisition mode. It is concluded that the EPID is useful for dosimetry purposes with 1% (1 SD) accuracy, bu t that the dose response relationship has to be determined for each EP ID and accelerator setting. (C) 1996 American Association of Physicist s in Medicine.