Influence of detector size in photon beam profile measurements

Correction is necessary to account for the detector size in clinical dosimetry of photon and electron beams. This correction is due to the absorbed dose gradient present in a finite-size detector. Further corrections are necessary when the detector and phantom materials are not the same. These corrections are due to the perturbation in the charged-particle fluence. Generally these corrections are applied to measurements along the central axis of the beam. Cross beam profile measurements, however, are not usually corrected for detector size. The ionization profile is also usually assumed to be equivalent to the absorbed dose profile. The authors have corrected the ionization chamber size effect by two approaches: extrapolation of measurements to zero detector size and deconvolution of measurements using a simple model for the detector response function. The authors have measured absorbed dose profiles to water using a small water-equivalent plastic scintillation detector. Film profile measurements also studied. The ionization profile corrected for detector size and absorbed dose profile were not equal, probably due to loss of charged-particle equilibrium in the beam edges.