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Photon beam quality variations of a flattening filter free linear accelerator

Georg, Dietmar; Kragl, Gabriele; Af Wetterstedt, Sacha; McCavana, Patrick; McClean, Brendan and Knöös, Tommy LU (2010) In Medical Physics 37(1). p.49-53
Abstract
Purpose: Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. Methods: Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland),... (More)
Purpose: Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. Methods: Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm). Results: For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta=10 degrees. These results agree within +/-2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV. Conclusions: Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3264617] (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
half value layer, unflattened beams, off-axis energy variation
in
Medical Physics
volume
37
issue
1
pages
49 - 53
publisher
American Association of Physicists in Medicine
external identifiers
  • wos:000273172100007
  • scopus:73649083321
ISSN
0094-2405
DOI
10.1118/1.3264617
language
English
LU publication?
yes
id
bec2e456-ac8f-4512-970e-06079d049cad (old id 1533632)
date added to LUP
2010-01-27 15:28:16
date last changed
2018-05-29 09:19:58
@article{bec2e456-ac8f-4512-970e-06079d049cad,
  abstract     = {Purpose: Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. Methods: Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm). Results: For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta=10 degrees. These results agree within +/-2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV. Conclusions: Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3264617]},
  author       = {Georg, Dietmar and Kragl, Gabriele and Af Wetterstedt, Sacha and McCavana, Patrick and McClean, Brendan and Knöös, Tommy},
  issn         = {0094-2405},
  keyword      = {half value layer,unflattened beams,off-axis energy variation},
  language     = {eng},
  number       = {1},
  pages        = {49--53},
  publisher    = {American Association of Physicists in Medicine},
  series       = {Medical Physics},
  title        = {Photon beam quality variations of a flattening filter free linear accelerator},
  url          = {http://dx.doi.org/10.1118/1.3264617},
  volume       = {37},
  year         = {2010},
}