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Title: On the effective point of measurement in megavoltage photon beams

Abstract

This paper presents a numerical investigation of the effective point of measurement of thimble ionization chambers in megavoltage photon beams using Monte Carlo simulations with the EGSNRC system. It is shown that the effective point of measurement for relative photon beam dosimetry depends on every detail of the chamber design, including the cavity length, the mass density of the wall material, and the size of the central electrode, in addition to the cavity radius. Moreover, the effective point of measurement also depends on the beam quality and the field size. The paper therefore argues that the upstream shift of 0.6 times the cavity radius, recommended in current dosimetry protocols, is inadequate for accurate relative photon beam dosimetry, particularly in the build-up region. On the other hand, once the effective point of measurement is selected appropriately, measured depth-ionization curves can be equated to measured depth-dose curves for all depths within {+-}0.5%.

Authors:
 [1]
  1. Ionizing Radiation Standards, NRC, Ottawa, K1A 0R6 (Canada)
Publication Date:
OSTI Identifier:
20853195
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 33; Journal Issue: 6; Other Information: DOI: 10.1118/1.2201147; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; COMPUTERIZED SIMULATION; DEPTH DOSE DISTRIBUTIONS; DIAGRAMS; DOSIMETRY; IONIZATION CHAMBERS; MONTE CARLO METHOD; PHOTON BEAMS

Citation Formats

Kawrakow, Iwan. On the effective point of measurement in megavoltage photon beams. United States: N. p., 2006. Web. doi:10.1118/1.2201147.
Kawrakow, Iwan. On the effective point of measurement in megavoltage photon beams. United States. https://doi.org/10.1118/1.2201147
Kawrakow, Iwan. 2006. "On the effective point of measurement in megavoltage photon beams". United States. https://doi.org/10.1118/1.2201147.
@article{osti_20853195,
title = {On the effective point of measurement in megavoltage photon beams},
author = {Kawrakow, Iwan},
abstractNote = {This paper presents a numerical investigation of the effective point of measurement of thimble ionization chambers in megavoltage photon beams using Monte Carlo simulations with the EGSNRC system. It is shown that the effective point of measurement for relative photon beam dosimetry depends on every detail of the chamber design, including the cavity length, the mass density of the wall material, and the size of the central electrode, in addition to the cavity radius. Moreover, the effective point of measurement also depends on the beam quality and the field size. The paper therefore argues that the upstream shift of 0.6 times the cavity radius, recommended in current dosimetry protocols, is inadequate for accurate relative photon beam dosimetry, particularly in the build-up region. On the other hand, once the effective point of measurement is selected appropriately, measured depth-ionization curves can be equated to measured depth-dose curves for all depths within {+-}0.5%.},
doi = {10.1118/1.2201147},
url = {https://www.osti.gov/biblio/20853195}, journal = {Medical Physics},
issn = {0094-2405},
number = 6,
volume = 33,
place = {United States},
year = {Thu Jun 15 00:00:00 EDT 2006},
month = {Thu Jun 15 00:00:00 EDT 2006}
}