RBE of quasi-monoenergetic 60 MeV neutron radiation for induction of dicentric chromosomes in human lymphocytes.

The production of dicentric chromosomes in human lymphocytes by high-energy neutron radiation was studied using a quasi-monoenergetic 60 MeV neutron beam. The average yield coefficient [see text] of the linear dose-response relationship for dicentric chromosomes was measured to be (0.146+/-0.016) Gy-1. This confirms our earlier observations that above 400 keV, the yield of dicentric chromosomes decreases with increasing neutron energy. Using the linear-quadratic dose-response relationship for dicentric chromosomes established in blood of the same donor for 60Co gamma-rays as a reference radiation, an average maximum low-dose RBE (RBEM) of 14+/-4 for 60 MeV quasi-monoenergetic neutrons with a dose-weighted average energy [see text] of 41.0 MeV is obtained. A correction procedure was applied, to account for the low-energy continuum of the quasi-monoenergetic spectral neutron distribution, and the yield coefficient alpha for 60 MeV neutrons was determined from the measured average yield coefficient [see text]. For alpha, a value of (0.115+/-0.026) Gy-1 was obtained corresponding to an RBEM of 11+/-4. The present experiments extend earlier investigations with monoenergetic neutrons to higher energies.

Quasi-monoenergetic neutron reference fields in the energy range from thermal to 200 MeV.

Well-characterised neutron fields are a prerequisite for the investigation of neutron detectors. Partly in collaboration with external partners, the PTB neutron metrology group makes available for other users neutron reference fields covering the full energy range from thermal to 200 MeV. The specification of the neutron fluence in these beams is traceable to primary standard cross sections.

Experimental kerma coefficients of biologically important materials at neutron energies below 75 MeV.

The present work summarizes our results already published on cross sections and partial kerma coefficients for hydrogen, carbon, and oxygen and then applies them for determining experimental partial and total kerma coefficients of composite biologically important materials. Double-differential cross sections for light-charged particle production (proton, deuteron, triton, and alpha particle) induced by fast neutrons on hydrogen, carbon, and oxygen have been experimentally measured at several incident energies from 25 to 75 MeV. The measurements covered the laboratory angular range 20 degrees to 160 degrees and were extended to very forward and very backward angles by using a reliable extrapolation procedure. Energy-differential, angle-differential, and total production cross sections were derived from the measured data. The experimental methods and data reduction procedures are briefly presented here. The experimental cross sections were compared to existing data in the literature for nucleon-induced reactions and against prediction of nuclear models. Partial and total elemental kerma coefficients were deduced on the basis of the measured cross sections. Procedures for extrapolating the partial kerma coefficients down to the reaction threshold energies for each of the measured ejectile species have been applied to carbon and oxygen. A simple-to-use analytical formula to describe the experimental hydrogen kerma coefficients was proposed which provides the recoil kerma coefficients in the incident neutron energy range 0.3 to 100 MeV. The present article reports for the first time experimental partial kerma coefficients for composite materials of biological interest. Resulting total kerma coefficients are compared to theoretical predictions and to other experimental data.

Experimental partial and total kerma coefficients for carbon deduced from microscopic cross sections at incident neutron energies below 75 MeV.

The double-differential cross sections for proton, deuteron, triton and alpha particle production in fast neutron induced reactions on carbon have been measured at several neutron energies from 25 to 75 MeV in the angular range from 20 degrees to 160 degrees. Experimental energy-differential, angle-differential and total cross sections are deduced, and consequently the partial and total kerma coefficients. The obtained experimental partial kerma coefficients are extrapolated down to the threshold energy of each measured reaction channel. Results on the experimental double differential, energy-differential and total cross sections are briefly presented. The deduced partial and total kerma coefficients of the present work are compared with previous measurements and theoretical predictions.

Experimental partial and total kerma coefficients for aluminium at incident neutron energies from reaction thresholds to 65 MeV.

For nine incident neutron energies in the range 25 to 65 MeV, the double-differential cross sections (energy spectra) for the (n, px), (n, dx), (n, tx) and (n, alpha x) reactions on aluminium have been measured at lab angles between 20 degrees and 160 degrees in steps of 10 degrees. The partial and total kerma coefficients have been determined from the experimental energy-differential cross sections. The resulting partial kerma coefficients for the incident neutron energy range of 25-65 MeV are extrapolated to the threshold energy of each registered reaction product type. Only illustrative results of the experimental double-differential, energy-differential and total cross sections are presented. The deduced partial and total kerma coefficients of the present work are compared with results of previous integral measurements and theoretical predictions.

A consistent set of neutron kerma coefficients from thermal to 150 MeV for biologically important materials.

Neutron cross sections for nonelastic and elastic reactions on a range of elements have been evaluated for incident energies up to 150 MeV. These cross sections agree well with experimental cross section data for charged-particle production as well as neutron and photon production. Therefore they can be used to determine kerma coefficients for calculations of energy deposition by neutrons in matter. Methods used to evaluate the neutron cross sections above 20 MeV, using nuclear model calculations and experimental data, are described. Below 20 MeV, the evaluated cross sections from the ENDF/B-VI library are adopted. Comparisons are shown between the evaluated charged-particle production cross sections and measured data. Kerma coefficients are derived from the neutron cross sections, for major isotopes of H, C, N, O, Al, Si, P, Ca, Fe, Cu, W, Pb, and for ICRU-muscle, A-150 tissue-equivalent plastic, and other compounds important for treatment planning and dosimetry. Numerous comparisons are made between our kerma coefficients and experimental kerma coefficient data, to validate our results, and agreement is found to be good. An important quantity in neutron dosimetry is the kerma coefficient ratio of ICRU-muscle to A-150 plastic. When this ratio is calculated from our kerma coefficient data, and averaged over the neutron energy spectra for higher-energy clinical therapy beams [three p (68) + Be beams, and a d (48.5) + Be beam], a value of 0.94 +/- 0.03 is obtained. Kerma ratios for water to A-150 plastic, and carbon to oxygen, are also compared with measurements where available.

Nuclear data for radiotherapy: presentation of a new ICRU report and IAEA initiatives.

An ICRU report entitled "Nuclear Data for Neutron and Proton Radiotherapy and for Radiation Protection" is in preparation. The present paper presents an overview of this report, along with examples of some of the results obtained for evaluated nuclear cross sections and kerma coefficients. These cross sections are evaluated using a combination of measured data and the GNASH nuclear model code for elements of importance for biological, dosimetric, beam modification and shielding purposes. In the case of hydrogen both R-matrix and phase-shift scattering theories are used. In the report neutron cross sections and kerma coefficients will be presented up to 150 MeV and proton cross sections up to 250 MeV. An IAEA Consultants' Meeting was also convened to examine the "Status of Nuclear Data needed for Radiation Therapy and Existing Data Development Activities in Member States". Recommendations were made regarding future endeavours.

Experimental hydrogen kerma factors for incident neutron energies from 25 to 75 MeV.

Hydrogen kerma factors and their uncertainties are deduced on an experimental basis, starting from our previously measured differential cross sections completed with available data from the literature, in the incident neutron energy range 25 to 75 MeV. The deduced experimental kerma factors are compared with theoretical predictions. A simple to use parametrization of the hydrogen kerma factor values in the incident neutron energy range 0.3 to 100 MeV is also proposed.

Experimental double-differential cross sections and derived kerma factors for oxygen at incident neutron energies from reaction thresholds to 65 MeV.

The double-differential cross sections (energy spectra) for the (n, px), (n, dx), (n, tx) and (n, rx) reactions on oxygen have been measured for nine incident neutron energies in the range 25 to 65 MeV at lab angles between 20 degrees and 160 degrees in steps of 10 degrees. From these measurements, the energy differential cross sections have been determined and consequently the partial and total kerma factors. Based on the obtained experimental partial kerma factors in the incident neutron energy range 25-65 MeV, a procedure is proposed for the extrapolation of these values to the reaction threshold energy of each measured reaction channel. Results of the experimental double-differential, energy differential and total cross sections are presented. The deduced partial and total kerma factors of the present work are compared with results of previous measurements and theoretical predictions.

The multileaf collimator for fast neutron therapy at Louvain-la-Neuve.

The multileaf collimator of the fast neutron therapy facility at Louvain-la-Neuve is described, as well as some of the physics experiments performed in order to evaluate the attenuation of neutron beams in different materials and thus optimize the composition of the collimator leaves. The multileaf collimator consists of two sets of 22 leaves each, which can be moved independently. They are made of iron and their thickness is 95 cm. Seven borated polyethylene disks are located in the distal part of the leaves in order to absorb more efficiently the low-energy component of the neutron spectrum. The width of the leaves is 1 cm at their distal part. The leaves can move 11 cm outwards and 6 cm inwards from their reference position, and field sizes up to 25.7 x 24.8 cm, as well as irregular field shapes, can be obtained. The inner part of the leaves and their two sides are always focused on the target. The complete multileaf collimator can rotate around the beam axis, from -90 degrees to +90 degrees from the reference position. The width of the penumbra (80-20% isodoses) is 0.64 cm and 1.17 cm at the depth of the maximum buildup and at 10 cm in depth respectively, for a 10 x 10 cm field size. The collimator is adequate for the energy of the p(65)+Be neutron beam of Louvain-la-Neuve and has been adapted to the fixed vertical beam. It has been designed following the original plans of Scanditronix, adjusted and fully assembled at the workshop of the Centre de Recherches du Cyclotron (CRC). Systematic measurements were performed in order to optimize the design and the composition of the leaves. In particular the attenuations of the actual beam and of monoenergetic neutron beams were measured in different materials such as iron and polyethylene. Above (upstream) the multileaf collimator, a fixed pre-collimator (iron thickness 50 cm; section 1 x 1 m) defines a conical aperture aligned on the largest opening of the leaves. It contains the two transmission chambers and a 2 cm thick polyethylene layer used for hardening the beam.

Kerma values deduced from neutron-induced charged-particle spectra of carbon from 40 to 75 MeV.

The double-differential cross-sections for the 12C(n, px), 12C(n, dx), 12C(n, tx) and 12C(n, alpha x) reactions have been measured at three incident neutron energies, 42.5, 62.7 and 72.8 MeV, at laboratory angles between 20 degrees and 160 degrees. From these measurements, the energy-differential cross-sections have been determined and consequently the partial and total kerma values. Results of the experimental double-differential and energy-differential cross-sections at the three incident neutron energies are presented. The deduced kerma values are compared with previous measurements and theoretical predictions.

New neutron sources for radiotherapy.

All existing neutron sources suffer from disadvantages which would not be tolerated in modern megavoltage X-ray equipment. Experimental work with 30-60 MeV protons on beryllium and other elements has shown how these difficulties can to a large extent be overcome. Angular distributions and kerma rates as a function of proton energy are presented for various targets. Thin targets are found to be a better method than filtration for improving the penetration of the beam. A 237Np fission counter and a GM counter have been used to decompose stray radiation into its neutron and gamma-ray components. A model of a treatment head was found to attenuate the neutron component to less than 1% of its value in the useful beam.

Improvement of a p(65)+Be neutron beam for therapy at Cyclone, Louvain-la-Neuve.

The variable energy cyclotron of the Catholic University of Louvain is used to produce intense neutron beams for neutron therapy purposes. As a first step, neutrons were produced by bombarding a Be target with 50 MeV deuterons; at present they are produced by 65 MeV protons. This paper describes the improvements to the target system. A new (17 mm) Be target together with the old (10 mm) Be target are inserted in a movable support which allows the production of neutrons either by 65 MeV protons or by 50 MeV deuterons. Both targets can be removed for proton beam therapy. The dosimetric characteristics of the p(65)+Be and d(50)+Be neutron beams are compared: dose rate, gamma-contribution, depth dose and room activation.

Comparison of neutron therapy beams produced by 50 MeV deuterons and 65 MeV protons on beryllium.

Neutron beams produced by bombarding a 10 cm thick beryllium target with 50 MeV deuterons have been used at Louvain-la-Neuve since nearly 4 years for routine therapeutic applications. At the end of 1981 they were replaced by neutron beams produced by 65 MeV protons on beryllium, mainly in order to improve the beam penetration in tissues. However, produced of neutrons from the p leads to Be reaction implies some disadvantages, mainly a lower dose rate and a higher activation level. In order to solve the problem a new target configuration was designed, consisting of a remote handled system which permits the use of 2 different target assemblies. The irradiated target is automatically removed immediately after the irradiation which greatly protects against exposure to the staff. The dosimetric characteristics of the d(50)-Be and p(65)-Be neutrons beams are compared.

Neutron beams from protons on beryllium.

Measurements of dose rate and penetration in water have been made for neutron beams produced by 30--75 MeV protons on beryllium. The effects of Polythene filters added on the target side of the collimator have also been studied. A neutron beam comparable with a photon beam from a 4--8 MeV linear accelerator can be produced with p/Be neutrons plus 5 cm Polythene filtrations, with protons in the range 50--75 MeV. This is a more economical method than use of the d/Be reaction.