Fig.1: Hanging inplane profiles for a 10x10 field. Gantry is on the right side.

Everybody was alarmed because we first thought the beam was really that asymmetric! But water phantom measurements quickly revealed that the true beam symmetry was nearly perfect. This is valid for all screenshots shown on this page: the real beam was always good ...

So the effect was probably originating from the image acquisition system. This was also confirmed when the images of the two identical linacs were compared: they showed the same effect.
The asymmetry is a little stronger for 6MV than for 15MV. In Crossplane direction, symmetry is ok. The effect depends on field size. For 25x25, or 27x36 (the calibration field size), the inplane symmetry is much better, but for smaller field sizes ... see Fig.2 and 3, comparing three 6MV inplane profiles. Gantry is on the right for all inplane profiles shown.

Fig.2: 6 MV inplane profiles for three different field sizes. For the 10x10 field, the profile hangs.

Fig.3: Beam Analysis of the three profiles shown in Fig.2. For 10x10, the measured beam asymmetry is nearly 3%.

Fig.4: A narrow and long field (see insert on the left). The imager was calibrated without beam profile correction. The plateau therefore should be perfectly flat.

Fig.5: Zoomed plateau in the 6MV image.

Fig.6: Inplane profiles of a 6 MV, 27x36 cm field taken at 3 (green), 30 (blue) and 300 MU (red).