PLANE-PARALLEL IONIZATION-CHAMBER RESPONSE IN THE BUILDUP REGION OF OBLIQUELY INCIDENT PHOTON BEAMS

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 e xhibit an even greater over-response in the buildup region of obliquel y incident photon beams. This over-response at oblique incidence is gr eatest at the surface of the phantom and increases with increasing ang le of beam incidence. In addition, the magnitude of the over-response depends on held size, beam energy, and chamber construction. This stud y shows that plane-parallel ionization chambers can over-respond by mo re than a factor of 2.3 at the phantom surface for obliquely incident high-energy photon fields. (C) 1997 American Association of Physicists in Medicine.