title: A perspex flattening filter for a 300k Vp orthovoltage X-ray beam.
abstract: For economic reasons modern equipment which produces low to medium energy X-rays covers the whole range of beam qualities from traditional superficial to orthovoltage radiation qualities. A recent trend shows an increasing number of installations of orthovoltage units in the cancer therapy community in the last five years. The use of a single anode for accelerating voltages between 60 and 300 kVp leads to compromises with regards to beam flatness and symmetry in the anode/cathode direction of the Siemens Stabilipan II radiation beam. A perspex flattening filter was designed to improve these beam parameters of the 300 kVp radiation beam (HVT 4mm Cu) at 50 cm and 60 cm FSD using a diaphragm-mounted field defining device. The filter design correlates with focal spot characteristics of the beam. The use of the filter improves flatness and symmetry for all measured field sizes from 6 x 6 cm2 to 18 x 18 cm2 by up to 8% (flatness) and 7% (symmetry) respectively. No significant difference in the depth dose characteristic of the 300 kVp beam was found with and without the filter. The only modification in the planning procedures required is the use of an attenuation factor of 0.89 for the filter. The use of the filter improves the dose distribution in treatment of patients undergoing orthovoltage radiotherapy--in particular treatments with large field sizes such as for metastases in the spinal column.
authors.affiliation: School of Applied Physics, University of South Australia, The Levels.
authors.forename: S
authors.initials: S
authors.lastname: Canaganayagam
authors.forename: W
authors.initials: W
authors.lastname: Hsu
authors.forename: S
authors.initials: S
authors.lastname: Ferguson
authors.forename: S
authors.initials: S
authors.lastname: Howlett
authors.forename: J
authors.initials: J
authors.lastname: Denham
authors.forename: T
authors.initials: T
authors.lastname: Kron
mesh_terms: D011856:Radiographic Image Enhancement / Q000295:instrumentation*