  Homogeneity for highly asymmetric fields has been studied for a Siemens
PRIMUS linear accelerator. The flattening filter has a radius smaller than the
primary collimator one, creating inhomogeneities that affect large fields in
areas far from the collimator axis, and asymmetric fields with large offset.
Profiles and absolute dose have been measured in fields with two jaws at
maximal position (20 cm) and the other two at maximal overtravel (10 cm.),
corresponding to 10 x 10 fields with extreme offset. Profiles have a remarkable
gradient decreasing towards the beam edge, making these fields unsuitable for
treatments. Results show that the design of the primary collimator and
flattening filter assembly has direct consequences in homogeneity. This can
have clinical consequences for treatments involving fields that include these
inhomogeneous areas. Comparisons with the treatment planning system (Philips
Pinnacle) calculations, that computes under the hypotheses of a uniformly
flattened beam, result in severe discrepancies.
