The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams.

In this paper the performance of the LA48 linear ion chamber array (PTW, Freiburg, Germany) for characterization of intensity-modulated (IM) beams was investigated. First, some elementary properties were explored. A series of beam penumbras and output factors for small rectangular fields were measured at 6 and 18 MV, and the results were compared with data obtained using a diamond detector. The energy and dose rate dependence of the array response were examined, and the leakage current was assessed. In a second step, profiles were measured for two clinically delivered IM beams and for a dynamic wedge. The interplay between the sharpening of the penumbra by the upper metal electrode plate of the array and the volume averaging of the 4 x 4 mm ion chamber elements results in precise measurements, even in regions of high dose gradient. It is true, however, that the metal electrodes imply a small energy spectrum dependence in the array response. The dose rate dependence is found to be negligible. All of this makes, the LA48 linear array a suitable device for analysing dose distributions of clinical IM beams.