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Absorbed Dose Determination and Characteristics of Degraded Electron Beams: Application to Intraoperative Radiation Therapy

Björk, Peter LU (2003)
Abstract (Swedish)
Popular Abstract in Swedish

Intraoperativ strålbehandling (IORT) innebär att en hög engångsstråldos ges direkt under operation till den kirurgiskt frilagda tumören och/eller kvarvarande tumörrester. Den vanligaste typen av strålning som används vid denna behandlingsform är elektronstrålning från konventionella medicinska linjäracceleratorer. Det krävs dock att ett speciellt applikatorsystem finns tillgängligt som kan begränsa strålfältet till önskad storlek samt skydda intilliggande frisk vävnad. Ett sådant applikatorsystem är geometriskt betydligt mer komplext än de applikatorer som används vid vanliga elektronbehandlingar. Detta får till följd att strålfältskarakteristiken förändras, vilket i sin tur kan påverka... (More)
Popular Abstract in Swedish

Intraoperativ strålbehandling (IORT) innebär att en hög engångsstråldos ges direkt under operation till den kirurgiskt frilagda tumören och/eller kvarvarande tumörrester. Den vanligaste typen av strålning som används vid denna behandlingsform är elektronstrålning från konventionella medicinska linjäracceleratorer. Det krävs dock att ett speciellt applikatorsystem finns tillgängligt som kan begränsa strålfältet till önskad storlek samt skydda intilliggande frisk vävnad. Ett sådant applikatorsystem är geometriskt betydligt mer komplext än de applikatorer som används vid vanliga elektronbehandlingar. Detta får till följd att strålfältskarakteristiken förändras, vilket i sin tur kan påverka bestämningen av absorberad dos i dessa strålfält. Vid strålbehandling av cancerpatienter är det mycket viktigt att den givna dosen ges med så stor noggrannhet som möjligt, eftersom detta i hög grad påverkar behandlingsresultatet. Syftet med detta arbete har varit att undersöka olika metoder för att förbättra bestämningen av absorberad dos i IORT-fält och andra typer av komplexa elektronstrålfält. Flera olika detektorsystem för mätning av den absorberade dosen har studerats. Omfattande datorberäkningar har utförts med så kallad Monte Carlo-teknik för att verifiera och förklara resultaten från de experimentella undersökningarna. Denna beräkningsteknik innebär att man kan följa enskilda strålpartiklars transport i en virtuell geometri och att den absorberade stråldosen kan studeras tredimensionellt. Dessutom erhålls ingående information som beskriver strålningsmiljön som detektorn utsätts för. Det här arbetet har visat att IORT-fält innehåller en betydligt större mängd lågenergetiska elektroner med större spridningsvinkel än vad vanliga elektronfält gör. Dessa elektroner kan påverka dels detektorresponsen, dels de parametrar som används för att omvandla detektorrespons till absorberad stråldos i patienten. Om konventionella mätmetoder (jonkammardosimetri enligt internationella rekommendationer) appliceras direkt på IORT-fallet införs ytterligare osäkerheter i dosbestämningen i storleksordningen 1%. Det rekommenderas i detta arbete att dosmätning istället bör ske med dioddetektorer eftersom osäkerheten då kan minskas, men också för att dessa detektorer är lätthanterliga mätinstrument som inte kräver omfattande korrektioner. (Less)
Abstract
The aim of this work was to quantify limitations of and uncertainties in commonly used dosimetric techniques for relative absorbed dose determination in degraded electron beams, such as those encountered in intraoperative radiation therapy (IORT) and small-electron-field radiotherapy. Three different detector types were investigated with regard to measurements of output factors and relative absorbed dose distributions: (1) a parallel-plate ionization chamber, (2) a p-type silicon diode detector and (3) a diamond detector. The Monte Carlo method was used to obtain a better understanding of the beam characteristics in the complex treatment situations. Comparisons were made with the beam characteristics in the reference geometry where the... (More)
The aim of this work was to quantify limitations of and uncertainties in commonly used dosimetric techniques for relative absorbed dose determination in degraded electron beams, such as those encountered in intraoperative radiation therapy (IORT) and small-electron-field radiotherapy. Three different detector types were investigated with regard to measurements of output factors and relative absorbed dose distributions: (1) a parallel-plate ionization chamber, (2) a p-type silicon diode detector and (3) a diamond detector. The Monte Carlo method was used to obtain a better understanding of the beam characteristics in the complex treatment situations. Comparisons were made with the beam characteristics in the reference geometry where the techniques for determining the absorbed dose are well described in international dosimetry protocols. The following results were obtained: (1) the IORT fields contained a considerably larger amount of scattered electrons than the reference field; (2) the IORT beams exhibited a broader energy spectrum and a wider angular distribution of the electrons at the phantom surface; and (3) the smaller the field size, the higher the mean energy at a certain depth in the phantom. These characteristics will change the radiation conditions at the measurement point, which can influence the detector signal as well as the mass collision stopping-power ratio between water and the detector material. This latter parameter is used in the general cavity equation to convert the mean absorbed dose in the detector to the absorbed dose in water. The diamond detector was shown to exhibit excellent properties for relative absorbed dose measurements in degraded electron beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of both output factors and relative absorbed dose distributions, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth, especially for nominal electron energies below 6 MeV. Application of ionization-chamber-based dosimetry will introduce uncertainties smaller than 0.3% in the output factor determination for conventional IORT beams. The IORT system at our department includes a 0.3 cm thick plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. The dosimetry protocols can be applied in a direct manner to obtain depth-dose distributions in degraded electron beams as the relative variation of the water-to-air stopping-power ratio is very similar for the reference field and the other investigated complex treatment situations. For practical reasons, the use of ion chambers for relative dosimetry in degraded beams is discouraged, due to the numerous corrections needed (e.g., for ion recombination, the polarity effect and stopping-power ratio variation). Instead, the readily available p-type silicon diode detector is recommended for relative absorbed dose measurements in complex electron fields. (Less)
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author
opponent
  • Prof. Karlsson, Mikael, Umeå University
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Clinical physics, radiology, tomography, medical instrumentation, Klinisk fysiologi, radiologi, tomografi, medicinsk instrumentering, absorbed dose distribution, output factor, stopping-power ratio, beam characteristics, Monte Carlo calculation, electron beam dosimetry, applicator system, radiation therapy, linear accelerator, intraoperative radiotherapy
pages
53 pages
publisher
Department of Radiation Physics, Lund university
defense location
Onkologiska klinikens föreläsningssal, Lund University Hospital
defense date
2003-10-10 10:15
ISBN
91-974444-2-1
language
English
LU publication?
yes
id
13efe6a9-c881-4bb1-95c9-8521086da541 (old id 466183)
date added to LUP
2007-09-10 10:43:51
date last changed
2016-09-19 08:45:12
@misc{13efe6a9-c881-4bb1-95c9-8521086da541,
  abstract     = {The aim of this work was to quantify limitations of and uncertainties in commonly used dosimetric techniques for relative absorbed dose determination in degraded electron beams, such as those encountered in intraoperative radiation therapy (IORT) and small-electron-field radiotherapy. Three different detector types were investigated with regard to measurements of output factors and relative absorbed dose distributions: (1) a parallel-plate ionization chamber, (2) a p-type silicon diode detector and (3) a diamond detector. The Monte Carlo method was used to obtain a better understanding of the beam characteristics in the complex treatment situations. Comparisons were made with the beam characteristics in the reference geometry where the techniques for determining the absorbed dose are well described in international dosimetry protocols. The following results were obtained: (1) the IORT fields contained a considerably larger amount of scattered electrons than the reference field; (2) the IORT beams exhibited a broader energy spectrum and a wider angular distribution of the electrons at the phantom surface; and (3) the smaller the field size, the higher the mean energy at a certain depth in the phantom. These characteristics will change the radiation conditions at the measurement point, which can influence the detector signal as well as the mass collision stopping-power ratio between water and the detector material. This latter parameter is used in the general cavity equation to convert the mean absorbed dose in the detector to the absorbed dose in water. The diamond detector was shown to exhibit excellent properties for relative absorbed dose measurements in degraded electron beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of both output factors and relative absorbed dose distributions, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth, especially for nominal electron energies below 6 MeV. Application of ionization-chamber-based dosimetry will introduce uncertainties smaller than 0.3% in the output factor determination for conventional IORT beams. The IORT system at our department includes a 0.3 cm thick plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. The dosimetry protocols can be applied in a direct manner to obtain depth-dose distributions in degraded electron beams as the relative variation of the water-to-air stopping-power ratio is very similar for the reference field and the other investigated complex treatment situations. For practical reasons, the use of ion chambers for relative dosimetry in degraded beams is discouraged, due to the numerous corrections needed (e.g., for ion recombination, the polarity effect and stopping-power ratio variation). Instead, the readily available p-type silicon diode detector is recommended for relative absorbed dose measurements in complex electron fields.},
  author       = {Björk, Peter},
  isbn         = {91-974444-2-1},
  keyword      = {Clinical physics,radiology,tomography,medical instrumentation,Klinisk fysiologi,radiologi,tomografi,medicinsk instrumentering,absorbed dose distribution,output factor,stopping-power ratio,beam characteristics,Monte Carlo calculation,electron beam dosimetry,applicator system,radiation therapy,linear accelerator,intraoperative radiotherapy},
  language     = {eng},
  pages        = {53},
  publisher    = {ARRAY(0x5d9e0e8)},
  title        = {Absorbed Dose Determination and Characteristics of Degraded Electron Beams: Application to Intraoperative Radiation Therapy},
  year         = {2003},
}