Field Symmetry And Flatness

Front 1

can be specified as a maximum permissible percentage variation from the average dose across the central 80% of the full width at half maximum of the beam profile in a plane transverse to beam axis

Back 1

flatness

Front 2

is usually specified for both transverse directions, each along the direction of collimator motion, for several field sizes, for a particular depth in a phantom, and for several gantry angles

Back 2

symmetry

Front 3

purpose

Back 3

to ensure that the symmetry and flatness of the radiation beam are at acceptable limits

Front 4

deviations may result in:

Back 4

unacceptable high or low dose regions in irradiated areas

Front 5

5 things that may result in unacceptable beam profiles

Back 5

changes in beam energy, beam alignment, bending magnet functions, target positions, and flattening filter selection

Front 6

tolerance for flatness:

non IMRT for photons:

non IMRT for electrons

IMRT for both electron and photon:

Back 6

2%

3%

1%

Front 7

tolerance for symmetry:

non IMRT:

IMRT:

Back 7

3%

1%