The introduction of dynamic treatment techniques in modern radiotherapy requires improvements of the conventional water phantom dosimetry for quality control. We have developed a system which consists of a scintillating screen (Gd₂O₂S:Tb) mounted on the beam exit side of a phantom and observed by a low noise CCD camera with long integration time. This system allows a measurement of the dose distribution in 2 dimensions simultaneously, which is very advantageous for dynamic systems.

Dynamic beam delivery systems are also of interest in protontherapy. We have tested this system at the first clinical system using such a technique: the 200 MeV proton gantry at the Paul Scherrer Institute in Switzerland. It uses the ‘spot-scanning’ method, in which the proton pencil beam is moved by a scanning magnet and table motion to cover the target volume. We have compared the CCD yield for a calibrated dose with earlier tests at the The Svedberg Laboratory in Uppsala, Sweden. The results obtained with the scanned pencil beam turned out to be consistent with those of the passive scattered proton beam within 1%. We have also measured homogeneous fields to look at the detection limits of beam positioning and preset errors. They are determined by the signal-to-noise ratio and well below the clinical maximum.

The greatest advantage of this system, however, is the capability of measuring more complex dose distributions. An example of such a distribution can be seen here. It is a dose distribution, in which the dose preset value varies linearly with the distance in the x-direction, comparable to a X-ray field with a wedge. With the PSI spot scanning system this sequence takes about 1.5 minute. This integration time can easily be reached by our system. It can be seen that the linearity of the system is very good. The sensitivity to small dose variations is clear from the detailed scanning structure, which becomes visible at high dose values.

The system demonstrates to be a useful tool for developing complicated dose distributions thanks to the fast availability of the results. We plan to enhance the system further in order to allow simultaneous 3D measurements.