title: Build-up cap materials for measurement of photon head-scatter factors.
abstract: The suitability of high-Z materials as build-up caps for head-scatter measurements has been investigated. Build-up caps are often used to enable characterization of fields too small for a mini-phantom. We have studied lead and brass build-up caps with sufficiently large wall thicknesses, as compared to the range of contaminating electrons originating in the accelerator head, and compared them with build-up caps made of ionization chamber equivalent materials, i.e. graphite. The results were also compared with measurements taken using square and cylindrical polystyrene mini-phantoms. Field sizes ranging from 3 cm x 3 cm up to 40 cm x 40 cm were studied for nominal photon energies of 4, 6, 10 and 18 MV. The results show that the use of lead and brass build-up caps produces normalized head-scatter data slightly different from graphite build-up caps for large fields at high photon energies. At lower energies, however, no significant differences were found. The intercomparison between the two different plastic mini-phantoms and graphite caps showed no differences.
authors.affiliation: Department of Radiation Physics, Lund University Hospital, Sweden.
authors.forename: L
authors.initials: L
authors.lastname: Weber
authors.forename: P
authors.initials: P
authors.lastname: Nilsson
authors.forename: A
authors.initials: A
authors.lastname: Ahnesjö
mesh_terms: D000497:Alloys; D003300:Copper; D006108:Graphite; D007854:Lead; D019047:Phantoms, Imaging; D017785:Photons; D019904:Polymethyl Methacrylate; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D015032:Zinc
chemical_list: D000497:Alloys; C048399:brass; D007854:Lead; D006108:Graphite; D003300:Copper; D019904:Polymethyl Methacrylate; D015032:Zinc