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Fast neutron absorbed dose distributions in the energy range 0.5-80 MeV: a Monte Carlo study
Linköping University, Department of Medical and Health Sciences, Radiation Physics. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Medical and Health Sciences, Radiation Physics. Linköping University, Faculty of Health Sciences. Östergötlands Läns Landsting, Centre of Surgery and Oncology, Department of Radiation Physics.ORCID iD: 0000-0003-0209-498X
2000 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 45, no 10, 2987-3007 p.Article in journal (Refereed) Published
Abstract [en]

Neutron pencil-beam absorbed dose distributions in phantoms of bone, ICRU soft tissue, muscle, adipose and the tissue substitutes water, A-150 (plastic) and PMMA (acrylic) have been calculated using the Monte Carlo code FLUKA in the energy range 0.5 to 80 MeV. For neutrons of energies ≤20 MeV, the results were compared to those obtained using the Monte Carlo code MCNP4B. Broad-beam depth doses and lateral dose distributions were derived. Broad-beam dose distributions in various materials were compared using two kinds of scaling factor: a depth-scaling factor and a dose-scaling factor. Build-up factors due to scattered neutrons and photons were derived and the appropriate choice of phantom material for determining dose distributions in soft tissue examined. Water was found to be a good substitute for soft tissue even at neutron energies as high as 80 MeV. The relative absorbed doses due to photons ranged from 2% to 15% for neutron energies 10-80 MeV depending on phantom material and depth. For neutron energies below 10 MeV the depth dose distributions derived with MCNP4B and FLUKA differed significantly, the difference being probably due to the use of multigroup transport of low energy (<19.6 MeV) neutrons in FLUKA. Agreement improved with increasing neutron energies up to 20 MeV. At energies >20 MeV, MCNP4B fails to describe dose build-up at the phantom interface and penumbra at the edge of the beam because it does not transport secondary charged particles. The penumbra width, defined as the distance between the 80% and 20% iso-dose levels at 5 cm depth and for a 10×10 cm2 field, was between 0.9 mm and 7.2 mm for neutron energies 10-80 MeV.

Place, publisher, year, edition, pages
2000. Vol. 45, no 10, 2987-3007 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-14360DOI: 10.1088/0031-9155/45/10/317ISI: 000089865300017OAI: oai:DiVA.org:liu-14360DiVA: diva2:23330
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2017-12-13
In thesis
1. Dosimetry and radiation quality in fast-neutron radiation therapy: A study of radiation quality and basic dosimetric properties of fast-neutrons for external beam radiotherapy and problems associated with corrections of measured charged particle cross-sections
Open this publication in new window or tab >>Dosimetry and radiation quality in fast-neutron radiation therapy: A study of radiation quality and basic dosimetric properties of fast-neutrons for external beam radiotherapy and problems associated with corrections of measured charged particle cross-sections
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dosimetric properties of fast-neutron beams with energies ≤80 MeV were explored using Monte Carlo techniques. Taking into account transport of all relevant types of released charged particles (electrons, protons, deuterons, tritons, 3He and α particles) pencil-beam dose distributions were derived and used to calculate absorbed dose distributions. Broad-beam depth doses in phantoms of different materials were calculated and compared and the scaling factors required for converting absorbed dose in one material to absorbed dose in another derived. The scaling factors were in good agreement with available published data and show that water is a good substitute for soft tissue even at neutron energies as high as 80 MeV. The inherent penumbra and the fraction of absorbed dose due to photon interactions were also studied, and found to be consistent with measured values reported in the literature.

Treatment planning in fast-neutron therapy is commonly performed using dose calculation algorithms designed for photon beam therapy. When applied to neutron beams, these algorithms have limitations arising from the physical models used. Monte Carlo derived neutron pencil-beam kernels were parameterized and implemented in the photon dose calculation algorithms of the TMS (MDS Nordion) treatment planning system. It was shown that these algorithms yield good results in homogeneous water media. However, the method used to calculate heterogeneity corrections in the photon dose calculation algorithm did not yield correct results for neutron beams in heterogeneous media.

To achieve results with adequate accuracy using Monte Carlo simulations, fundamental cross-section data are needed. Neutron cross-sections are still not sufficiently well known. At the The Svedberg Laboratory in Uppsala, Sweden, an experimental facility has been designed to measure neutron-induced charged-particle production cross-sections for (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xα) reactions at neutron energies up to 100 MeV. Depending on neutron energy, these generated particles account for up to 90% of the absorbed dose. In experimental determination of the cross-sections, measured data have to be corrected for the energies lost by the charged particles before leaving the target in which they were generated. To correct for the energy-losses, a computational code (CRAWL) was developed. It uses a stripping method. With the limitation of reduced energy resolution, spectra derived using CRAWL compares well with those derived using other methods.

In fast-neutron therapy, the relative biological effectiveness (RBE) varies from 1.5 to 5, depending on neutron energy, dose level and biological end-point. LET and other physical quantities, developed within the field of microdosimetry over the past couple of decades, have been used to describe RBE variations between different fast-neutron beams as well as within a neutron irradiated body. In this work, a Monte Carlo code (SHIELD-HIT) capable of transporting all charged particles contributing to absorbed dose, was used to calculate energy-differential charged particle spectra. Using these spectra, values of the RBE related quantities LD, γD, γ* and R were derived and studied as function of neutron energy, phantom material and position in a phantom. Reasonable agreement with measured data in the literature was found and indicates that the quantities may be used to predict RBE variations in an arbitrary fast-neutron beam.

Place, publisher, year, edition, pages
Institutionen för medicin och vård, 2007
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 989
Keyword
Neutron, Dosimetry, Radiotherapy, Monte Carlo, Microdosimetry, Cross-section, RBE, LET, Energy-loss corrections
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:liu:diva-8589 (URN)978-91-85715-37-4 (ISBN)
Public defence
2007-04-04, Conrad, Röntgenavdelningen, Campus US, Linköpings Universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2015-03-20
2. Fast neutron dosimetry: a study of basic dosimetric properties of fast-neutrons for external beam radiotherapy and problems associated with corrections of measured charged particle cross-sections
Open this publication in new window or tab >>Fast neutron dosimetry: a study of basic dosimetric properties of fast-neutrons for external beam radiotherapy and problems associated with corrections of measured charged particle cross-sections
2001 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Basic dosimetric properties of fast-neutron beams with energies ≤80 MeV were explored using Monte Carlo techniques. Elementary pencil-beam dose distributions taking into account transport of all relevant types of released charged particles (protons, deuterons, tritons, 3He and a particles) were calculated and used to derive several absorbed dose distributions. Broad-beam depth doses in phantoms of different materials were compared and scaling factors calculated to convert absorbed dose in one material to absorbed dose in another. The scaling factors were in good agreement with available published data and show that water is a good substitute for soft tissue even at neutron energies as high as 80 Me V. The inherent penumbra and fraction of absorbed dose due to photons were also studied, and found to be consistent with published values.

Treatment planning in fast-neutron therapy is commonly performed using dose calculation algorithms designed for photon beam therapy. These algorithms have limitations in the physical models when applied to neutron beams. Monte Carlo derived neutron pencil-beam kernels were parameterized and implemented into the photon dose calculation algorithms of the TMS (MDS Nordion) treatment planning system. It was shown that these algorithms yield good results in homogeneous water media. However, the heterogeneity correction method of the photon dose calculation algorithm failed to calculate correct results in heterogeneous media for neutron beams.

Fundamental cross-section data are needed when calculating absorbed doses. To achieve results with adequate accuracy, neutron cross-sections are still not sufficiently well known. At the The Svedberg Laboratory in Uppsala, Sweden, an experimental facility has been designed to measure neutron-induced charged-particle production cross-sections for (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xα) reactions at neutron energies up to 100 MeV. In order to derive the energy distributions of charged particles generated inside the production target, the measured data have to be corrected for the energy lost by the particles in the target. In this work a code (CRAWL) was developed for the reconstruction of the true spectrum. It uses a stripping method. With the limitation of reduced energy resolution, results using CRAWL compare well with those of other methods.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2001. 44 p.
Series
Linköping Studies in Health Sciences. Thesis, ISSN 1100-6013 ; 48
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-27572 (URN)12235 (Local ID)91-7219-975-X (ISBN)12235 (Archive number)12235 (OAI)
Presentation
2013-06-05, Ögonklinikens föreläsningssal, Universitetssjukhuset, Linköping, 09:15 (Swedish)
Opponent
Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2013-07-10

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Söderberg, JonasAlm Carlsson, Gudrun

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