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Estimation of neutron dose contributions to personnel working around high-energy medical linear accelerators for radiation therapy

Muca, Drita (2006)
Medical Physics Programme
Abstract
Purpose: Medical linear accelerators that operate above 10 MeV produce neutrons by photonuclear reactions which present a potential radiation hazard to the personnel. The purpose of this study was to estimate neutron dose contributions to the personnel working with external radiotherapy at Malmö University Hospital (UMAS), compare different kind of neutron detectors/dosimeters, and evaluate how to translate the results of the measurements to an effective dose. By estimating the neutron doses received by the personnel one can decide if better shielding is required.Method: Two accelerators that operate above 10 MeV, Varian Clinac 2100 C/D and Elekta precise, were investigated. Measurements with area monitoring devices (BF3 and 3He based... (More)
Purpose: Medical linear accelerators that operate above 10 MeV produce neutrons by photonuclear reactions which present a potential radiation hazard to the personnel. The purpose of this study was to estimate neutron dose contributions to the personnel working with external radiotherapy at Malmö University Hospital (UMAS), compare different kind of neutron detectors/dosimeters, and evaluate how to translate the results of the measurements to an effective dose. By estimating the neutron doses received by the personnel one can decide if better shielding is required.Method: Two accelerators that operate above 10 MeV, Varian Clinac 2100 C/D and Elekta precise, were investigated. Measurements with area monitoring devices (BF3 and 3He based instruments) were performed outside the treatment room (in experimental form), to make a survey of the neutron dose equivalent rate around the radiotherapy facility. Measurements were also performed inside the control rooms when the accelerators were in clinical use (during patient treatment). The personnel were carrying different kind of personal neutron dosemeters (electronic dosemeters, bubble detectors and etched-track detectors) during their time of work. The relation between the operational quantities and the protection quantities were studied.Results and Discussion: It was found that the highest neutron dose rates outside the treatment room when irradiating a phantom with 18 MV photon beams from the Elekta machine, were outside the treatment door (~150 μSv/h), at the hallway between the control rooms (~90 μSv/h), and inside one of the control rooms (~40 μSv/h). Furthermore from this machine, the neutron contribution to the measured dose equivalent rate was higher than the photon component. The estimated neutron dose equivalent varied up to a factor of two and occasionally even more for the different measuring devices. The highest personal dose equivalent from the personal dosemeters was estimated to be in the order of about1 mSv/year. The personal dose equivalent is the most appropriate operational quantity for estimating the effective dose while the ambient dose equivalent is a rough approximation of the effective dose.Conclusions: This study shows that the neutron component from the bunker with the Elekta machine needs to be considered when high-energy photon beams are used. The measure that could be taken in order to reduce the neutron dose to the personnel is to avoid 18 MV treatments in that bunker. Another solution is to build an additional “neutron stopping door” with hydrogen-containing shielding material inside the treatment room that has to be closed during 18 MV treatments. (Less)
Abstract (Swedish)
Strålning kan medföra både skada och nytta. Inom strålbehandling utnyttjas strålningen för att bota eller lindra cancersjukdomar. Målet är att med hjälp av joniserande strålning slå ut alla tumörceller samtidigt som man försöker skona övriga friska celler. Strålningen som produceras i den vanligaste utrustningen för extern strålbehandling, nämligen linjäraccelerator, är elektronstrålning och fotonstrålning av varierande energi. Strålslag och energi bestäms beroende på hur djupt in i kroppen man vill att strålningen ska tränga in.

För att kunna behandla en djupt liggande tumör används normalt högenergetisk fotonstrålning. Vid dessa höga fotonenergier genereras oönskade neutroner och produktionen ökar med fotonenergin. Neutronerna... (More)
Strålning kan medföra både skada och nytta. Inom strålbehandling utnyttjas strålningen för att bota eller lindra cancersjukdomar. Målet är att med hjälp av joniserande strålning slå ut alla tumörceller samtidigt som man försöker skona övriga friska celler. Strålningen som produceras i den vanligaste utrustningen för extern strålbehandling, nämligen linjäraccelerator, är elektronstrålning och fotonstrålning av varierande energi. Strålslag och energi bestäms beroende på hur djupt in i kroppen man vill att strålningen ska tränga in.

För att kunna behandla en djupt liggande tumör används normalt högenergetisk fotonstrålning. Vid dessa höga fotonenergier genereras oönskade neutroner och produktionen ökar med fotonenergin. Neutronerna stannar inte bara i behandlingsrummet utan kan gå igenom väggar och dörrar, som i första hand är avsedda som strålskydd för fotoner och elektroner. Neutroner ger därmed stråldos till personal.

Strålmiljön runt linjäracceleratorerna berör personal som arbetar med behandlingarna. För att kontrollera stråldosen till personal bär de en så kallad dosimeter som mäter hur stor stråldos användaren har utsatts för. Dessa dosimetrar mäter normalt bara fotondosbidraget och inte dosbidraget från neutroner.

Målet med detta arbete var att kartlägga neutrondosbidraget till personalen som arbetar med strålbehandling i Universitetssjukhuset MAS i Malmö. Olika typer av neutron-mätinstrument användes och jämfördes. Mätningar gjorde dels i form av experimentella mätningar och dels när acceleratorn var i klinisk drift vid patientbehandlingar.

Resultaten visade högre bidrag till stråldosen från neutroner än från fotoner från ett av behandlingsrummen vid behandling med högsta fotonenergin. Neutronbidraget får inte försummas och de åtgärder som kan göras är antingen att inte göra behandlingar med högsta fotonenergin på just den acceleratorn eller om detta inte är praktiskt möjligt, bygga en extra ”neutrondörr” inne i behandlingsrummet. (Less)
Please use this url to cite or link to this publication:
author
Muca, Drita
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Strålterapi
language
English
id
2157001
date added to LUP
2011-09-13 11:48:31
date last changed
2011-12-06 14:41:52
@misc{2157001,
  abstract     = {Purpose: Medical linear accelerators that operate above 10 MeV produce neutrons by photonuclear reactions which present a potential radiation hazard to the personnel. The purpose of this study was to estimate neutron dose contributions to the personnel working with external radiotherapy at Malmö University Hospital (UMAS), compare different kind of neutron detectors/dosimeters, and evaluate how to translate the results of the measurements to an effective dose. By estimating the neutron doses received by the personnel one can decide if better shielding is required.Method: Two accelerators that operate above 10 MeV, Varian Clinac 2100 C/D and Elekta precise, were investigated. Measurements with area monitoring devices (BF3 and 3He based instruments) were performed outside the treatment room (in experimental form), to make a survey of the neutron dose equivalent rate around the radiotherapy facility. Measurements were also performed inside the control rooms when the accelerators were in clinical use (during patient treatment). The personnel were carrying different kind of personal neutron dosemeters (electronic dosemeters, bubble detectors and etched-track detectors) during their time of work. The relation between the operational quantities and the protection quantities were studied.Results and Discussion: It was found that the highest neutron dose rates outside the treatment room when irradiating a phantom with 18 MV photon beams from the Elekta machine, were outside the treatment door (~150 μSv/h), at the hallway between the control rooms (~90 μSv/h), and inside one of the control rooms (~40 μSv/h). Furthermore from this machine, the neutron contribution to the measured dose equivalent rate was higher than the photon component. The estimated neutron dose equivalent varied up to a factor of two and occasionally even more for the different measuring devices. The highest personal dose equivalent from the personal dosemeters was estimated to be in the order of about1 mSv/year. The personal dose equivalent is the most appropriate operational quantity for estimating the effective dose while the ambient dose equivalent is a rough approximation of the effective dose.Conclusions: This study shows that the neutron component from the bunker with the Elekta machine needs to be considered when high-energy photon beams are used. The measure that could be taken in order to reduce the neutron dose to the personnel is to avoid 18 MV treatments in that bunker. Another solution is to build an additional “neutron stopping door” with hydrogen-containing shielding material inside the treatment room that has to be closed during 18 MV treatments.},
  author       = {Muca, Drita},
  keyword      = {Strålterapi},
  language     = {eng},
  note         = {Student Paper},
  title        = {Estimation of neutron dose contributions to personnel working around high-energy medical linear accelerators for radiation therapy},
  year         = {2006},
}