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Dosimetric effects of removing the flattening filter in radiotherapy treatment units

Dalaryd, Mårten LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

Vid extern strålbehandling används en s.k. linjäraccelerator för att producera och leverera den önskade strålningen till cancertumörer. I linjäracceleratorn accelereras elektroner till nära ljusets hastighet och styrs sedan mot en metalplatta där de bromsas upp och genererar bromsstrålningsfotoner (högenergetisk röntgenstrålning). Intensiteten av den strålning som sänds ut är störst i den riktning som elektronerna haft, det vill säga mitt i det fält som genereras, och för att generera ett strålfält med lika hög intensitet överallt placeras ett konformat utjämningsfilter i strålfältet. Filtret ger dock upphov till vissa nackdelar och ett homogent strålfält är idag inte nödvändigt för att leverera... (More)
Popular Abstract in Swedish

Vid extern strålbehandling används en s.k. linjäraccelerator för att producera och leverera den önskade strålningen till cancertumörer. I linjäracceleratorn accelereras elektroner till nära ljusets hastighet och styrs sedan mot en metalplatta där de bromsas upp och genererar bromsstrålningsfotoner (högenergetisk röntgenstrålning). Intensiteten av den strålning som sänds ut är störst i den riktning som elektronerna haft, det vill säga mitt i det fält som genereras, och för att generera ett strålfält med lika hög intensitet överallt placeras ett konformat utjämningsfilter i strålfältet. Filtret ger dock upphov till vissa nackdelar och ett homogent strålfält är idag inte nödvändigt för att leverera vissa typer av strålbehandlingar.

I det här arbetet har egenskaper hos en linjäraccelerator utan utjämningsfilter undersökts. Istället för filtret placerades antingen en tunn koppar- eller järnplatta i strålfältet, vilket är en nödvändighet för att kunna kontrollera strålfältet på ett säkert sätt.

Mätningar och datorberäkningar med så kallad Monte Carlo-teknik, både av det nya strålfältet samt av konventionella strålfält med utjämningsfilter har genomförts, för att ta reda på vilka skillnader som finns i den levererade strålningen. Denna nya behandlingsteknik levererar strålningen med dubbelt så hög intensitet centralt i strålfältet, något som kan leda till kortare behandlingstider. Den ger också upphov till mindre spridd strålning och mindre transmission genom de metallblock som formar strålfältet, vilket kan minska onödig bestrålning av patienten.

Det har inte varit bekräftat hur väl man kan mäta den absorberade dosen från kliniska fotonfält utan utjämningsfilter enligt internationella rekommendationer för jonkammardosimetri. I detta arbete utvärderades hur väl en viktig parameter för dessa mätningar kan förutsägas när stålkvalitetsmått som främst är framtagna för fält med utjämningsfilter används för kliniska strålfält utan utjämningsfilter. Två nya stålkvalitetsmått undersöktes också, vilka visade sig vara mer noggranna än de som rekommenderas internationellt. (Less)
Abstract
The aim of this work was to investigate the dosimetric effects of removing the flattening filter from conventional C-arm medical linear accelerators. In conventional linear accelerators used for radiotherapy, a flattening filter is positioned in the beam line to provide a uniform lateral dose profile at a specified depth in water. However, for some radiotherapy treatments, a uniform lateral dose profile is not necessary, e.g. stereotactic treatments with small fields or treatments with intensity modulated fields.

In this work, a comprehensive set of measurements and Monte Carlo simulations for a modified Elekta Precise linear accelerator, operating with and without a flattening filter, were performed and the differences were... (More)
The aim of this work was to investigate the dosimetric effects of removing the flattening filter from conventional C-arm medical linear accelerators. In conventional linear accelerators used for radiotherapy, a flattening filter is positioned in the beam line to provide a uniform lateral dose profile at a specified depth in water. However, for some radiotherapy treatments, a uniform lateral dose profile is not necessary, e.g. stereotactic treatments with small fields or treatments with intensity modulated fields.

In this work, a comprehensive set of measurements and Monte Carlo simulations for a modified Elekta Precise linear accelerator, operating with and without a flattening filter, were performed and the differences were evaluated. For an Elekta Precise linac, it was found that by removing the flattening filter the dose could be delivered approximately twice as fast as when the flattening filter is in the beam line, under certain conditions. The scatter produced in the treatment head was reduced by 30 %–45 % when the flattening filter was removed and the variation of scattered radiation with field size was also reduced. Removal of the flattening filter resulted in a softer photon energy spectra which leads to a steeper absorbed dose fall-off with depth and less lateral variation across the field. By increasing the acceleration potential of the linac, the depth–dose profiles become more similar to those of the equivalent conventional photon beam and thus the output will also be increased.

The suitability of two beam quality measures, TPR20,10 and %dd(10)x, in predicting water to air mass collision stopping-power ratios sw,air for flattening filter-free photon beams was also investigated. These quality measures are used in reference dosimetry for the determination of absorbed dose in water. It was shown that the relationship between TPR20,10 and sw,air used in a current international code of practice for reference dosimetry, overestimates the stopping-power ratio by approximately 0.3 % for flattening filter-free photon beams, while the relationship between %dd(10)x and sw,air, used in the North American code of practice is more accurate. A new beam quality metric, consisting of both TPR20,10 and TPR10,5 was evaluated. It was found that this new beam quality specifier more accurately predicted stopping power ratios for flattening filter-free photon beams. A beam quality specifier defined by the first two moments (describing the mean and variance) of the spectral distribution was also investigated and found to accurately predict stopping-power ratios for beams without a flattening filter. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Montelius, Anders, Uppsala University, Department of Radiology, Oncology and Radiation Science, Uppsala, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
pages
94 pages
publisher
Department of Medical Radiation Physics, Clinical Sciences, Lund, Lund University
defense location
Lecure hall, 3rd floor in the radiotherapy building, Skåne University Hospital, Klinikgatan 5, 22185 Lund
defense date
2015-09-18 13:00
ISBN
978-91-7623-421-1 (pdf)
978-91-7623-420-4 (print)
language
English
LU publication?
yes
id
37d0be6b-1211-4c31-8624-f2a5636744e8 (old id 7761467)
date added to LUP
2015-09-04 16:51:57
date last changed
2016-09-19 08:45:07
@misc{37d0be6b-1211-4c31-8624-f2a5636744e8,
  abstract     = {The aim of this work was to investigate the dosimetric effects of removing the flattening filter from conventional C-arm medical linear accelerators. In conventional linear accelerators used for radiotherapy, a flattening filter is positioned in the beam line to provide a uniform lateral dose profile at a specified depth in water. However, for some radiotherapy treatments, a uniform lateral dose profile is not necessary, e.g. stereotactic treatments with small fields or treatments with intensity modulated fields.<br/><br>
In this work, a comprehensive set of measurements and Monte Carlo simulations for a modified Elekta Precise linear accelerator, operating with and without a flattening filter, were performed and the differences were evaluated. For an Elekta Precise linac, it was found that by removing the flattening filter the dose could be delivered approximately twice as fast as when the flattening filter is in the beam line, under certain conditions. The scatter produced in the treatment head was reduced by 30 %–45 % when the flattening filter was removed and the variation of scattered radiation with field size was also reduced. Removal of the flattening filter resulted in a softer photon energy spectra which leads to a steeper absorbed dose fall-off with depth and less lateral variation across the field. By increasing the acceleration potential of the linac, the depth–dose profiles become more similar to those of the equivalent conventional photon beam and thus the output will also be increased.<br/><br>
The suitability of two beam quality measures, TPR20,10 and %dd(10)x, in predicting water to air mass collision stopping-power ratios sw,air for flattening filter-free photon beams was also investigated. These quality measures are used in reference dosimetry for the determination of absorbed dose in water. It was shown that the relationship between TPR20,10 and sw,air used in a current international code of practice for reference dosimetry, overestimates the stopping-power ratio by approximately 0.3 % for flattening filter-free photon beams, while the relationship between %dd(10)x and sw,air, used in the North American code of practice is more accurate. A new beam quality metric, consisting of both TPR20,10 and TPR10,5 was evaluated. It was found that this new beam quality specifier more accurately predicted stopping power ratios for flattening filter-free photon beams. A beam quality specifier defined by the first two moments (describing the mean and variance) of the spectral distribution was also investigated and found to accurately predict stopping-power ratios for beams without a flattening filter.},
  author       = {Dalaryd, Mårten},
  isbn         = {978-91-7623-421-1 (pdf)},
  language     = {eng},
  pages        = {94},
  publisher    = {ARRAY(0x5cadb08)},
  title        = {Dosimetric effects of removing the flattening filter in radiotherapy treatment units},
  year         = {2015},
}