## Question

How is the Beam Symmetry calculated in the myQA Daily?

## Answer

Please see here a layout of the chambers in the myQA Daily:

There are 25 and 57 chambers for the symmetry measurement for field size, 10 x 10 cm² and 20 x 20 cm², respectively.

Beam symmetry Sx and Sy is calculated from the uniformity- and kTP -corrected measurement signals Mi,j of the chambers marked in the image above for the two reference fields. Indices i and j denote the chamber position in X and Y direction, with i = j = 0 at the center position.

For crossline symmetry, the ratio RX,i,j = Mi,j / M−i,j is calculated from the measurement signals of each chamber and its mirrored counterpart. The maximum ratio in percent is the measurement value for field symmetry in crossline direction: SX = 100% · max{ RX,i,j }.

Similarly, the field-symmetry in inline direction SY = 100% · max{ RY,i,j }, where RY,i,j = Mi,j / Mi,−j.