Characteristics of a spoiled 6-MV beam from a dual-energy linear accelerator.

A beam spoiler for a 6-MV x-ray beam was designed and spoiled beam dosimetry performed. The spoiled beam quality was similar to that of the unspoiled beam. The percentage depth doses at 5 mm were at least 90% for all field sizes except 4 x 4 cm, while the surface dose at 0 mm varied from 22.5% for a 4 x 4-cm field to 85.8% for a 22 x 22-cm field. The spoiled beam showed degraded beam flatness for small field size at superficial depths; the beam flatness at 6 mm for a 4 x 4-cm field was +/- 9.0%. The beam flatness improved as the field size increased, e.g., at same depth, the beam flatness was +/- 2.5% for a 22 x 22-cm field. The penumbra width (90%-10%) of the spoiled beam was greater than that of the unspoiled beam for all field sizes and at all depths; e.g., at 0-mm depth, for a 22 x 22-cm field, the penumbra width was 3.8 cm for the unspoiled beam and 7.2 cm for the spoiled beam. Beyond dmax the difference in the penumbra widths between the spoiled and unspoiled beam was about 3-4 mm for all field sizes and at all depths. The peripheral dose was larger for the spoiled beam.