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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
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.
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 [en]
Neutron, Dosimetry, Radiotherapy, Monte Carlo, Microdosimetry, Cross-section, RBE, LET, Energy-loss corrections
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:liu:diva-8589ISBN: 978-91-85715-37-4OAI: oai:DiVA.org:liu-8589DiVA: diva2:23333
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
List of papers
1. Correction of measured charged-particle spectra for energy losses in the target: A comparison of three methods
Open this publication in new window or tab >>Correction of measured charged-particle spectra for energy losses in the target: A comparison of three methods
2002 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, Vol. 195, no 3-4, 426-434 p.Article in journal (Refereed) Published
Abstract [en]

The experimental facility, MEDLEY, at the The Svedberg Laboratory in Uppsala, has been constructed 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. Corrections for the energy loss of the charged particles in the target are needed in these measurements, as well as for loss of particles. Different approaches have been used in the literature to solve this problem. In this work, a stripping method is developed, which is compared with other methods developed by Rezentes et al. and Slypen et al. The results obtained using the three codes are similar and they could all be used for correction of experimental charged-particle spectra. Statistical fluctuations in the measured spectra cause problems independent of the applied technique, but the way to handle it differs in the three codes.

Keyword
Neutron, Cross-section, Charged particle, Energy-loss corrections
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14359 (URN)10.1016/S0168-583X(02)01090-X (DOI)000178915500023 ()
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2015-03-20
2. Fast neutron absorbed dose distributions in the energy range 0.5-80 MeV: a Monte Carlo study
Open this publication in new window or tab >>Fast neutron absorbed dose distributions in the energy range 0.5-80 MeV: a Monte Carlo study
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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14360 (URN)10.1088/0031-9155/45/10/317 (DOI)000089865300017 ()
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2015-03-20
3. Monte Carlo evaluation of a photon pencil kernel algorithm applied to fast neutron therapy treatment planning
Open this publication in new window or tab >>Monte Carlo evaluation of a photon pencil kernel algorithm applied to fast neutron therapy treatment planning
2003 (English)In: Physics in Medicine and Biology, ISSN 0031-9155 (print) 1361-6560 (online), Vol. 48, no 20, 3327-3344 p.Article in journal (Refereed) Published
Abstract [en]

When dedicated software is lacking, treatment planning for fast neutron therapy is sometimes performed using dose calculation algorithms designed for photon beam therapy. In this work Monte Carlo derived neutron pencil kernels in water were parametrized using the photon dose algorithm implemented in the Nucletron TMS (treatment management system) treatment planning system. A rectangular fast-neutron fluence spectrum with energies 0–40 MeV (resembling a polyethylene filtered p(41)+ Be spectrum) was used. Central axis depth doses and lateral dose distributions were calculated and compared with the corresponding dose distributions from Monte Carlo calculations for homogeneous water and heterogeneous slab phantoms. All absorbed doses were normalized to the reference dose at 10 cm depth for a field of radius 5.6 cm in a 30 × 40 × 20 cm3 water test phantom. Agreement to within 7% was found in both the lateral and the depth dose distributions. The deviations could be explained as due to differences in size between the test phantom and that used in deriving the pencil kernel (radius 200 cm, thickness 50 cm). In the heterogeneous phantom, the TMS, with a directly applied neutron pencil kernel, and Monte Carlo calculated absorbed doses agree approximately for muscle but show large deviations for media such as adipose or bone. For the latter media, agreement was substantially improved by correcting the absorbed doses calculated in TMS with the neutron kerma factor ratio and the stopping power ratio between tissue and water. The multipurpose Monte Carlo code FLUKA was used both in calculating the pencil kernel and in direct calculations of absorbed dose in the phantom.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14361 (URN)10.1088/0031-9155/48/20/005 (DOI)
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2015-03-20
4. RBE related quantities in fast-neutron therapy beams derived using Monte Carlo calculated charged particle spectra
Open this publication in new window or tab >>RBE related quantities in fast-neutron therapy beams derived using Monte Carlo calculated charged particle spectra
Show others...
Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-14362 (URN)
Available from: 2007-03-22 Created: 2007-03-22 Last updated: 2010-01-13

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