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 dos imetry performed. The spoiled beam quality was similar to that of the unspoiled beam. The percentage depth doses at 5 mm were at least 90% f or all field sizes except 4 X 4 cm, while the surface dose at 0 mm var ied from 22.5% for a 4 X 4-cm field to 85.8% for a 22 x 22-cm field. T he 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 fiel d. 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 c m for the unspoiled beam and 7.2 cm for the spoiled beam. Beyond d(max ) the difference in the penumbra widths between the spoiled and unspoi led beam was about 3-4 mm for all field sizes and at all depths. The p eripheral dose was larger for the spoiled beam.