Monte Carlo calculations of correction factors for plane‐parallel ionization chambers in clinical electron dosimetry

Abstract

Recent standard dosimetry protocols recommend that plane-parallel ionization chambers be used in the measurements of depth-dose distributions or the calibration of low-energy electron beams with beam quality R50 <4 g/cm2. In electron dosimetry protocols with the plane-parallel chambers, the wall correction factor, Pwall, in water is assumed to be unity and the replacement correction factor, Prepl, is taken to be unity for well-guarded plane-parallel chambers, at all measurement depths. This study calculated Pwall and Prepl for NACP-02, Markus, and Roos plane-parallel chambers in clinical electron dosimetry using the EGSnrc Monte Carlo code system. The Pwall values for the plane-parallel chambers increased rapidly as a function of depth in water, especially at lower energy. The value around R50 for NACP-02 was about 10% greater than unity at 4 MeV. The effect was smaller for higher electron energies. Similarly, Prepl values with depth increased drastically at the region with the steep dose gradient for lower energy. For Markus Prepl departed more than 10% from unity close to R50 due to the narrow guard ring width. Prepl for NACP-02 and Roos was close to unity in the plateau region of depth-dose curves that includes a reference depth, dref. It was also found that the ratio of the dose to water and the dose to the sensitive volume in the air cavity for the plane-parallel chambers, Dw/[Dair]pp, at d(ref) differs significantly from that assumed by electron dosimetry protocols.

On the wall perturbation correction for a parallel‐plate NACP‐02 chamber in clinical electron beams

“…At larger depths, due to the blurring of the spectral electron fluence, also the total fluence is strongly changing even with small chamber shifts. Therefore, neglecting the nonwater equivalence of the entrance window, wall perturbation corrections of 10% and more at the half value depth may result for low electron energies, as also reported for example by Buckley and Rogers, 14 Araki,11 or Verhaegen et al 13 Shifting the NACP-02 chamber by an amount ⌬z = −0.058 cm toward the focus, the electron fluence within the chamber and the bare cavity bears the largest resemblance independent of the depth within the water phantom and independent of the energy of the primary electrons. The resulting depth dependence of p wall is within 1% even for the lowest electron energy used in this study.…”

“…The presence of an energy and depth dependence of perturbation corrections for parallel-plate chambers in clinical electron dosimetry has been shown in recent years by several authors. 13,14,11,12,22 These investigations have also demonstrated that the calculated perturbation corrections are quite sensitive to small variations in the positioning of the parallelplate chamber, especially at depths larger than the reference depth z ref .…”

“…The entrance windows of plane-parallel chambers are made of thin foils; therefore, the wall correction factor p wall is assumed to be unity in all current dosimetry protocols 1-5 independent of electron energy and depth. It has been shown experimentally [6][7][8][9][10] as well as by Monte Carlo simulations [11][12][13][14][15][16][17] that this assumption may be invalid.…”

Technical Note: EGSnrc‐based dosimetric study of the BEBIG HDR brachytherapy sources

“…[6][7][8][9][10][11][12] In this work, the EGSnrcMP code system 5 is used to calculate twodimensional ͑2D͒ dose rate distributions around the BEBIG 60 Co HDR sources in an unbounded liquid water phantom in Cartesian format. The calculated dose rate data are compared to GEANT4-based published data.…”

Characterization of a synthetic single crystal diamond Schottky diode for radiotherapy electron beam dosimetry

“…6(a) by both SCDD and Si-D. Recent papers [33][34][35][36] showed that perturbation factors in electron beams can be nonunity also for "well-guarded" plane-parallel ionization chambers and their values depend on the measurement depth. Such a dependence should be accounted for when measuring depth dose distributions.…”