Real time 2D verification tools for IMRT

One of the most widely used methods for IMRT verification is to compare the dose distribution calculated by Treatment Planning Systems (TPS) in a simple-geometry phantom with the measured dose distribution with films [7.1, 7.2, 7.3, 7.4, 7.5]. If films are normalized to additional ionization chamber measurements, the dose distribution can be expressed in absolute value. A check of these modulated beam fluence patterns have also been carried out using a fast-beam 2D-beam imaging system [7.6]. All these methods have their strength in the very good spatial resolution and, when are properly calibrated, in the good precision obtained for the dose measurements. On the other hand they can be extremely cumbersome and time consuming. Othervise, the use of EPIDs as devices for beam-dosimetry analysis, are also investigated by many authors [7.7, 7.8, 7.9, 7.10]. Since some time, tools that allow a 2D verification with matrices of ionization chambers or silicon diodes are available on the market. In this case, the response is immediately available in a digital form and can be compared to the TPS predictions. The spatial resolution and number of sampling points are inferior to those obtained with films. The goal of the present tests was to compare the behavior of two commercial devices and one detector, a prototype that has been developed at the University and INFN Torino. The three detectors have been tested at different Hospitals, to be able to measure their performances when IMRT fields are delivered with different accelerators, multileaf collimators (MLC) and using different TPSs.