The use of an electronic portal imaging device for exit dosimetry

An electronic portal imaging device (Portal Vision, Varian, PV) is in use at the Regional Hospital of Varese (Italy), providing near real time images of patient anatomy within x-ray fields during treatment on linear accelerators. The possibility of using PV as a dosemeter of the exit dose was assessed. The relationship between the digital PV reading in a fixed point in air on the beam axis and the dose-rate in the same point measured with a ionization chamber was investigated. The measurements were obtained irradiating a polystyrene phantom and placing the PV and a ionization chamber at a distance of 30 cm from the exit surface of the phantom and varying the phantom thickness, field dimensions and linac dose-rate. A square root function was used to fit the data. The good quality of the fitting demonstrates the possibility of using PV reading as a dose-rate measurement. The relationship between PV and the exit dose (the dose at the exit surface of the patient) was studied. PV readings at 30 cm from the exit surface of the same phantom was directly related to the exit dose. In this case too, the best function fitting the data is a square root function. That shows the possibility of using PV not only as a localization device but as a dose-verification tool too. The off-axis response was also examined, comparing PV images to those obtained with films at the exit point dose in the same irradiation conditions. To approach patient conditions, PV was also tested with phantoms containing some inhomogeneity areas. (Aluminium and Air cavities) and with curved-shaped phantoms