title: Demonstration of a combined filter to improve the field uniformity of a 90 kV superficial X-ray therapy machine for different treatment field sizes.
abstract: The field uniformity of a superficial X-ray machine operating at 90 kV, 10 mA and filtered with a 1.1 mm aluminium beam hardening filter was investigated at a depth of 1 cm below the surface. Uniformity measurements were carried out using films and a densitometer to detect the relative absorbed dose across the field. Film dosimetry was assessed by comparison with ionization chamber dosimetry in a water tank. The original flat hardening aluminium filter was replaced by a combined, profiled filter to improve the uniformity across field sizes 20 cm, 5 cm and 2 cm diameter as well as hardening the beam. Flatness of the beam profile was improved for the 20 cm field size from +/-7.5% to +/-1.3% across the anode/cathode direction and from +/-7.9% to +/-4.7% in the anode/cathode direction. For the 5 cm field size the improvement was from +/-4% to +/-3% and from +/-5.3% to +/-3.6% and for 2 cm field size from +/-3.4% to +/-2.8% and from 10.5% to +/-9.7% in the same directions, respectively. Beam quality measurements were made and the original half-value-layer was reduced from 2.21 +/- 0.09 mm to 2.07 +/- 0.09 mm. The project 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.
authors.affiliation: Coventry University, School of Natural and Environmental Sciences, UK.
authors.forename: J
authors.initials: J
authors.lastname: Meyer
authors.forename: J A
authors.initials: JA
authors.lastname: Mills
mesh_terms: D005373:Film Dosimetry; D005374:Filtration / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D011834:Radiation Monitoring / Q000379:methods*; D011878:Radiotherapy / Q000295:instrumentation* / Q000592:standards