title: Plane-parallel ionization chamber response in the buildup region of obliquely incident photon beams.
abstract: Fixed-separation plane-parallel ionization chambers have been shown to overestimate the dose in the buildup region of normally incident high-energy photon beams. This work shows that these ionization chambers exhibit an even greater over-response in the buildup region of obliquely incident photon beams. This over-response at oblique incidence is greatest at the surface of the phantom and increases with increasing angle of beam incidence. In addition, the magnitude of the over-response depends on field size, beam energy, and chamber construction. This study shows that plane-parallel ionization chambers can over-respond by more than a factor of 2.3 at the phantom surface for obliquely incident high-energy photon fields.
authors.affiliation: Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota, Minneapolis 55455, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Gerbi
authors.forename: F M
authors.initials: FM
authors.lastname: Khan
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D006801:Humans; D019047:Phantoms, Imaging; D017785:Photons; D011874:Radiometry / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy; D013819:Thermoluminescent Dosimetry