Use of a tungsten filter to improve beam uniformity.

A general method for calculating the dose distribution in an irradiated volume is to evaluate the primary and scatter components separately according to the method described by Clarkson and Cunningham. It was found, however, that for a 6 MV Siemens accelerator the calculated dose overestimated the peripheral dose at depths beyond 10 cm by 3%-6%. The difference was attributed to the varying beam quality across the field. This beam quality variation was decreased by hardening the beam with a permanently installed 1/3 in. tungsten filter inserted between the beam flattening filter and the mirror base assembly. The tungsten filter had a more pronounced beam hardening effect at the beam edge than along the central ray. For example, the dose rate in air at the beam edge for a 30 x 30 cm2 field was 13% higher than along the central ray without the tungsten filter. The addition of the 1/8 in. filter decreased this horn to 6%. The beam quality along the central ray also increased. The tissue-air ratio for zero field size along the central ray increased by 2% with the addition of the tungsten filter. The scatter-air ratio, however, did not change with the added filter. Agreement within 1%-2% was achieved between the calculated and measured beam profiles at all depths in a phantom when the tungsten filter was added.