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Tissue-phantom ratios from percentage depth doses

Bjärngard, Bengt E; Zhu, Timothy C and Ceberg, Crister LU (1996) In Medical Physics 23(5). p.629-634
Abstract
When converting fractional (percentage) depth doses to tissue-phantom ratios, one must use a factor that accounts for the different source-to-point distances. Two minor correction factors are also involved. One is the ratio of total to primary dose at the two different distances from the source, for the same depth and field size. This factor is usually ignored. It was determined experimentally that this can introduce up to 1.5% error at 6 MV. The second correction factor reflects differences related to scattered photons and electrons at the depth of normalization in the two geometries. This correction is accounted for in published conversion procedures. It was found to be less than 1% provided the normalization depth is sufficient for... (More)
When converting fractional (percentage) depth doses to tissue-phantom ratios, one must use a factor that accounts for the different source-to-point distances. Two minor correction factors are also involved. One is the ratio of total to primary dose at the two different distances from the source, for the same depth and field size. This factor is usually ignored. It was determined experimentally that this can introduce up to 1.5% error at 6 MV. The second correction factor reflects differences related to scattered photons and electrons at the depth of normalization in the two geometries. This correction is accounted for in published conversion procedures. It was found to be less than 1% provided the normalization depth is sufficient for electron equilibrium, which occurs first well beyond the depth of maximum dose. One may avoid electron-equilibrium problems by using an interim normalization depth that provides electron equilibrium with some margin, renormalizing to a shallower depth if desired. With this precaution, the accuracy when measured fractional depth doses were converted to tissue-phantom ratios was comparable to that of directly measured tissue-phantom ratios even when the correction factors were ignored. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Medical Physics
volume
23
issue
5
pages
629 - 634
publisher
American Association of Physicists in Medicine
external identifiers
  • pmid:8724733
  • scopus:0029930762
ISSN
0094-2405
DOI
10.1118/1.597698
language
English
LU publication?
yes
id
e7fc67de-5dc8-42db-91b3-361ff5d905cc (old id 1110740)
date added to LUP
2008-07-22 08:26:17
date last changed
2017-07-23 04:39:11
@article{e7fc67de-5dc8-42db-91b3-361ff5d905cc,
  abstract     = {When converting fractional (percentage) depth doses to tissue-phantom ratios, one must use a factor that accounts for the different source-to-point distances. Two minor correction factors are also involved. One is the ratio of total to primary dose at the two different distances from the source, for the same depth and field size. This factor is usually ignored. It was determined experimentally that this can introduce up to 1.5% error at 6 MV. The second correction factor reflects differences related to scattered photons and electrons at the depth of normalization in the two geometries. This correction is accounted for in published conversion procedures. It was found to be less than 1% provided the normalization depth is sufficient for electron equilibrium, which occurs first well beyond the depth of maximum dose. One may avoid electron-equilibrium problems by using an interim normalization depth that provides electron equilibrium with some margin, renormalizing to a shallower depth if desired. With this precaution, the accuracy when measured fractional depth doses were converted to tissue-phantom ratios was comparable to that of directly measured tissue-phantom ratios even when the correction factors were ignored.},
  author       = {Bjärngard, Bengt E and Zhu, Timothy C and Ceberg, Crister},
  issn         = {0094-2405},
  language     = {eng},
  number       = {5},
  pages        = {629--634},
  publisher    = {American Association of Physicists in Medicine},
  series       = {Medical Physics},
  title        = {Tissue-phantom ratios from percentage depth doses},
  url          = {http://dx.doi.org/10.1118/1.597698},
  volume       = {23},
  year         = {1996},
}