A theoretical design of a flattening filter to improve field uniformity of a superficial therapeutic X-ray beam.

A Monte Carlo model has been developed using the MCNP code to aid the design of a flattening filter, to improve field uniformity of a superficial x-ray machine. The machine is operating at 90 kV and filtered with a 1.1 mm aluminium filter. In the theoretical simulation the original flat filter was replaced by a varying thickness filter to improve the uniformity across field sizes 20, 5 and 2 cm diameter as well as hardening the beam. Simulation results showed that flatness of the beam profile was improved for the 20 cm field size from +/- 7.3% to +/- 1.1% across the anode/cathode direction and from +/- 7.7% to +/- 3.2% in the anode/cathode direction. For the 5 cm field size the improvement was from +/- 4.6% to +/- 3.1% and from +/- 5.5% to +/- 3.4%, and for the 2 cm field size from +/- 3.1% to +/- 2.4% and from +/- 10.2% to +/- 9.5%, in the same directions, respectively. Beam quality simulations were made and the original half-value layer was reduced from 2.21 +/- 0.09 mm aluminium to 2.04 +/- 0.09 mm aluminium. The study demonstrated that it was possible to build a filter capable of flattening the beam profile for different sized applicators without significantly changing the penetrating ability of the beam.