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Application of the convolution method for calculation of output factors for therapy photon beams

Ahnesjö, Anders; Knöös, Tommy LU and Montelius, Anders (1992) In Medical Physics 19(2). p.295-301
Abstract
The output factor for a therapy photon beam is defined as the dose per monitor unit relative to the dose per monitor unit in a reference field. Convolution models for photon dose calculations yield the dose in units normalized to the incident energy fluence with phantom scatter intrinsically modeled. Output factors calculated with the convolution method as the dose per unit energy fluence relative to the calculated dose per unit energy fluence in a reference field could deviate as much as 5% if corrections are not made for perturbations due to treatment head scatter. Significant perturbations are particles backscattered from the collimators to the monitor and photons forward scattered from the filter and collimators in the treatment head.... (More)
The output factor for a therapy photon beam is defined as the dose per monitor unit relative to the dose per monitor unit in a reference field. Convolution models for photon dose calculations yield the dose in units normalized to the incident energy fluence with phantom scatter intrinsically modeled. Output factors calculated with the convolution method as the dose per unit energy fluence relative to the calculated dose per unit energy fluence in a reference field could deviate as much as 5% if corrections are not made for perturbations due to treatment head scatter. Significant perturbations are particles backscattered from the collimators to the monitor and photons forward scattered from the filter and collimators in the treatment head. The forward scatter adds an "unmonitored" contribution to the total energy fluence of the beam. A model is developed that describes the field size dependence of these perturbations for conversion of output factors, calculated with the convolution method, to machine output factors as an integrated part in treatment planning. The necessary machine characteristics are derived from measurements of the output in air for a limited set of field sizes. The method has been tested using five different multileaf collimated irregular fields at 6 MV and for a large set of rectangular fields at 5, 6, and 18 MV and found to predict output factors with an accuracy better than 1%. (Less)
Please use this url to cite or link to this publication:
author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Medical Physics
volume
19
issue
2
pages
295 - 301
publisher
American Association of Physicists in Medicine
external identifiers
  • pmid:1584120
  • scopus:0026697038
ISSN
0094-2405
DOI
10.1118/1.596859
language
English
LU publication?
yes
id
c97def15-1f23-4003-bb7c-f3b635b1d555 (old id 1106381)
date added to LUP
2008-07-30 11:07:24
date last changed
2017-08-06 04:40:22
@article{c97def15-1f23-4003-bb7c-f3b635b1d555,
  abstract     = {The output factor for a therapy photon beam is defined as the dose per monitor unit relative to the dose per monitor unit in a reference field. Convolution models for photon dose calculations yield the dose in units normalized to the incident energy fluence with phantom scatter intrinsically modeled. Output factors calculated with the convolution method as the dose per unit energy fluence relative to the calculated dose per unit energy fluence in a reference field could deviate as much as 5% if corrections are not made for perturbations due to treatment head scatter. Significant perturbations are particles backscattered from the collimators to the monitor and photons forward scattered from the filter and collimators in the treatment head. The forward scatter adds an "unmonitored" contribution to the total energy fluence of the beam. A model is developed that describes the field size dependence of these perturbations for conversion of output factors, calculated with the convolution method, to machine output factors as an integrated part in treatment planning. The necessary machine characteristics are derived from measurements of the output in air for a limited set of field sizes. The method has been tested using five different multileaf collimated irregular fields at 6 MV and for a large set of rectangular fields at 5, 6, and 18 MV and found to predict output factors with an accuracy better than 1%.},
  author       = {Ahnesjö, Anders and Knöös, Tommy and Montelius, Anders},
  issn         = {0094-2405},
  language     = {eng},
  number       = {2},
  pages        = {295--301},
  publisher    = {American Association of Physicists in Medicine},
  series       = {Medical Physics},
  title        = {Application of the convolution method for calculation of output factors for therapy photon beams},
  url          = {http://dx.doi.org/10.1118/1.596859},
  volume       = {19},
  year         = {1992},
}