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Development and evaluation of an independent system for absorbed dose calculations in radiotherapy

Johnsson, Stefan LU (2003)
Abstract (Swedish)
Popular Abstract in Swedish

Strålbehandling av cancerpatienter sker idag med avancerad teknik och utrustning. Förberedelserna inför behandlingen är omfattande och kräver medverkan av flera olika personalkategorier och övervakning via datoriserade verifikationssystem. I varje led i förberedelsekedjan finns en potentiell risk för avvikelser, vilka kan vara både systematiska eller tillfälliga i sin natur. Den senare typen av avvikelser orsakas ofta av rena misstag på grund av den ”mänskliga faktorn”. För att undvika felaktigheter där patienten kan komma till skada, är det av största vikt att alla inställningar kontrolleras vid första behandlingstillfället. Framför allt är det viktigt att verifiera att den givna stråldosen... (More)
Popular Abstract in Swedish

Strålbehandling av cancerpatienter sker idag med avancerad teknik och utrustning. Förberedelserna inför behandlingen är omfattande och kräver medverkan av flera olika personalkategorier och övervakning via datoriserade verifikationssystem. I varje led i förberedelsekedjan finns en potentiell risk för avvikelser, vilka kan vara både systematiska eller tillfälliga i sin natur. Den senare typen av avvikelser orsakas ofta av rena misstag på grund av den ”mänskliga faktorn”. För att undvika felaktigheter där patienten kan komma till skada, är det av största vikt att alla inställningar kontrolleras vid första behandlingstillfället. Framför allt är det viktigt att verifiera att den givna stråldosen överensstämmer med den planerade. Inom ramen för detta arbete har ett kvalitetssäkringsprogram bestående av ett komplett system för oberoende stråldosberäkningar utvecklats. Den beräkningsmodell som används är helt fristående från det kommersiella system som normalt används för att beräkna stråldosen. Modellen bygger på ett fåtal mätbara parametrar som är direkt kopplade till strålkvaliteten på den aktuella behandingsmaskinen. Just parametrarnas koppling till strålkvalitet och hur dessa parametrar erhålls från mätningar har varit utgånsgpunkten för flera av delarbetena. Trots sin enkelhet klarar modellen av att beräkna stråldoser i mycket avancerade patientgeometrier med en hög noggranhet. Beräkningsmodellens enkelhet gör vidare att den är möjlig att implementera i små handdatorer, vilka kan medföras ute i strålbehandlingskliniken. Dessa handdatorer kan i sin tur kommunicera med behandlingsmaskinernas egna kontrollsystem, vilket gör att man alltid har tillgång till den senaste patientinformationen. Systemet har använts kliniskt under en längre tid och i den efterföljande utvärderingen kunde ett systematiskt fel i patientgeometrin för vissa patientgrupper identifieras. Inga tillfälliga fel upptäcktes. Vidare framgick det att den aktionsnivå som tidigare satts var för hög vid vissa typer av behandlingar. Utvärderingen visade också att handdatorn är ett utmärkt verktyg med vilken man kan utföra oberoende, avancerade och effektiva kontroller av stråldosen till patienen vid första behandlingstillfället. Systemet används numera rutinmässigt på strålbehandlingskliniken vid Lunds Universitetssjukhus och på Radioterapikliniken vid Rigshospitalet i Köpenhamn och kommer inom kort också att implementeras på andra kliniker i Sverige. (Less)
Abstract
The aim of this work was to develop, implement and evaluate an independent system with which to calculate the absorbed dose, delivered by high-energy X-ray beams, to the prescription point and the depth of dose maximum. The introduction of such a system in the clinical routine may help ensure high-quality treatment and avoidance of errors which may jeopardise the clinical outcome of the treatment (i.e. under- or overdose). A set of equations for calculating the absorbed dose to the prescription point was compiled in a software application (“HandCalc”), which is completely independent of the treatment planning system (TPS). For instance, HandCalc includes models to calculate the absorbed dose from photons scattered in the patient, the... (More)
The aim of this work was to develop, implement and evaluate an independent system with which to calculate the absorbed dose, delivered by high-energy X-ray beams, to the prescription point and the depth of dose maximum. The introduction of such a system in the clinical routine may help ensure high-quality treatment and avoidance of errors which may jeopardise the clinical outcome of the treatment (i.e. under- or overdose). A set of equations for calculating the absorbed dose to the prescription point was compiled in a software application (“HandCalc”), which is completely independent of the treatment planning system (TPS). For instance, HandCalc includes models to calculate the absorbed dose from photons scattered in the patient, the transmission of the primary kerma in the patient, the variation of the primary kerma in air with collimator setting (i.e. head scatter), and corrections for heterogeneities in the patient. A new expression for the transmission of the primary kerma in the patient was derived in which the coefficients are strictly defined (and given a physical interpretation) by the first two moments of the spectral distribution of the incident beam. Further investigations also revealed that these moments can be used to determine water-to-air stopping power ratios more accurately than other beam quality indices. In practice, the moments are derived from “in-air equivalent”, narrow-beam measurements using a mini-phantom. The degree of in-air equivalence was investigated with Monte Carlo simulations, which showed that the optimum measurement depth in a mini-phantom is somewhat below the depth of dose maximum. Based upon comparisons with measurements and the TPS, a clinical action level of +/- 4% was chosen for HandCalc. Deviations greater than this are, with all probability, due to erroneous handling of the patient dataset during the preparation phase. An “entrance dose factor” was added in order to correct the dose calculations at the depth of dose maximum where electron equilibrium has not been established. The entrance dose factor was found to vary with beam quality and collimator setting, while no variation was detected with the presence of an acrylic tray (for block support) or with the source-surface distance (SSD). HandCalc was implemented in a hand-held PC which makes dose calculations inside the treatment room at the time of administration of the first fraction possible. An important feature of HandCalc is the built-in report function, which logs results from the calculation for later evaluation. In a study including 700 patients, deviations greater than the action level were found to be due either to limitations in HandCalc or to a systematic deviation between the planned and measured SSD. HandCalc has proven to be a fast and accurate tool for independent dose calculations inside the treatment room and it requires only a limited amount of extra time for the user to perform the calculations. Thus, it can easily be incorporated as part of the daily clinical quality control programme in order to prevent errors which may jeopardise the clinical outcome of the treatment. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Prof. Svensson, Hans, Umeå, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
keywords
medicinsk instrumentering, tomografi, radiologi, Klinisk fysiologi, tomography, medical instrumentation, radiology, Clinical physics, cancer, Cytologi, onkologi, cancerology, oncology, Cytology, PDA, mini-phantom, in-air equivalence, primary kerma, transmission measurement, stopping power ratio, entrance dose, beam quality, quality control, error prevention, monitor unit calculation, radiation therapy, dose calculation
pages
59 pages
publisher
Stefan Johnsson (request by e-mail),
defense location
Onkologiska Klinikens Föreläsningssal, Lund
defense date
2003-03-21 10:15
ISBN
91-628-5498-4
language
English
LU publication?
yes
id
d88df1d0-b635-471f-81ab-db682b63260d (old id 465452)
date added to LUP
2007-09-27 09:58:27
date last changed
2016-09-19 08:45:02
@phdthesis{d88df1d0-b635-471f-81ab-db682b63260d,
  abstract     = {The aim of this work was to develop, implement and evaluate an independent system with which to calculate the absorbed dose, delivered by high-energy X-ray beams, to the prescription point and the depth of dose maximum. The introduction of such a system in the clinical routine may help ensure high-quality treatment and avoidance of errors which may jeopardise the clinical outcome of the treatment (i.e. under- or overdose). A set of equations for calculating the absorbed dose to the prescription point was compiled in a software application (“HandCalc”), which is completely independent of the treatment planning system (TPS). For instance, HandCalc includes models to calculate the absorbed dose from photons scattered in the patient, the transmission of the primary kerma in the patient, the variation of the primary kerma in air with collimator setting (i.e. head scatter), and corrections for heterogeneities in the patient. A new expression for the transmission of the primary kerma in the patient was derived in which the coefficients are strictly defined (and given a physical interpretation) by the first two moments of the spectral distribution of the incident beam. Further investigations also revealed that these moments can be used to determine water-to-air stopping power ratios more accurately than other beam quality indices. In practice, the moments are derived from “in-air equivalent”, narrow-beam measurements using a mini-phantom. The degree of in-air equivalence was investigated with Monte Carlo simulations, which showed that the optimum measurement depth in a mini-phantom is somewhat below the depth of dose maximum. Based upon comparisons with measurements and the TPS, a clinical action level of +/- 4% was chosen for HandCalc. Deviations greater than this are, with all probability, due to erroneous handling of the patient dataset during the preparation phase. An “entrance dose factor” was added in order to correct the dose calculations at the depth of dose maximum where electron equilibrium has not been established. The entrance dose factor was found to vary with beam quality and collimator setting, while no variation was detected with the presence of an acrylic tray (for block support) or with the source-surface distance (SSD). HandCalc was implemented in a hand-held PC which makes dose calculations inside the treatment room at the time of administration of the first fraction possible. An important feature of HandCalc is the built-in report function, which logs results from the calculation for later evaluation. In a study including 700 patients, deviations greater than the action level were found to be due either to limitations in HandCalc or to a systematic deviation between the planned and measured SSD. HandCalc has proven to be a fast and accurate tool for independent dose calculations inside the treatment room and it requires only a limited amount of extra time for the user to perform the calculations. Thus, it can easily be incorporated as part of the daily clinical quality control programme in order to prevent errors which may jeopardise the clinical outcome of the treatment.},
  author       = {Johnsson, Stefan},
  isbn         = {91-628-5498-4},
  keyword      = {medicinsk instrumentering,tomografi,radiologi,Klinisk fysiologi,tomography,medical instrumentation,radiology,Clinical physics,cancer,Cytologi,onkologi,cancerology,oncology,Cytology,PDA,mini-phantom,in-air equivalence,primary kerma,transmission measurement,stopping power ratio,entrance dose,beam quality,quality control,error prevention,monitor unit calculation,radiation therapy,dose calculation},
  language     = {eng},
  pages        = {59},
  publisher    = {Stefan Johnsson (request by e-mail),},
  school       = {Lund University},
  title        = {Development and evaluation of an independent system for absorbed dose calculations in radiotherapy},
  year         = {2003},
}