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At least 19 recordsLinked to original sources
PubMed2000

Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: applications for neutron capture therapy.

252Cf is a neutron emitting radioisotope which has promise for both standard brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the thermal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of 252Cf applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed manner on multiple workstations using MCNP. Three studies were executed: (1) relative phi(th) as a function of distance from a 252Cf AT source in an A-150 tissue equivalent plastic phantom using thermoluminescent dosimeters (TLDs) of varying 6Li/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two 252Cf AT sources, and (3) calculations of the impact of phantom material composition (e.g., A-150, water, brain, muscle) on phi(th) from moderated 252Cf fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed within 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measurements followed the ASTM E 262-86e protocol, and the ratio of activated plain and Cd encased gold foils (7.31) agreed well with the calculated ratio (7.26). Measured phi(th) at 5 cm (1.70+/-0.10 x 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 x 10(7) n cm(-2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 24% less, respectively, at a distance of 5 cm from the source in a 15 cm diameter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 x 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm.

Algorithms↗
PubMed2000

Modification of the University of Washington Neutron Radiotherapy Facility for optimization of neutron capture enhanced fast-neutron therapy.

A modified neutron production target assembly has been developed to provide improved performance of the proton-cyclotron-based neutron radiotherapy facility at the University of Washington for applications involving neutron capture enhanced fast-neutron therapy. The new target produces a neutron beam that yields essentially the same fast-neutron physical depth-dose distribution as is produced by the current UW clinical system, but that also has an increased fraction of BNCT enhancement relative to the total therapeutic dose. The modified target is composed of a 5-millimeter layer of beryllium, followed by a 2.5-millimeter layer of tungsten, with a water-cooled copper backing. Measurements of the free-field neutron spectrum of the beam produced by the new target were performed using activation foils with a direct spectral unfolding technique. Water phantom measurements were performed using a tissue-equivalent ion chamber to characterize the fast-neutron depth-dose curve and sodium activation in soda-lime glass beads to characterize the thermal-neutron flux (and thus the expected neutron capture dose enhancement) as a function of depth. The results of the various measurements were quite consistent with expectations based on the design calculations for the modified target. The spectrum of the neutron beam produced by the new target features an enhanced low-energy flux component relative to the spectrum of the beam produced by the standard UW target. However, it has essentially the same high-energy neutron flux, with a reduced flux component in the mid-range of the energy spectrum. As a result, the measured physical depth-dose curve in a large water phantom has the same shape compared to the case of the standard UW clinical beam, but approximately twice the level of BNCT enhancement per unit background neutron dose at depths of clinical interest. In-vivo clinical testing of BNCT-enhanced fast-neutron therapy for canine lung tumors using the new beam was recently initiated.

Animals↗
PubMed2003

Relative biological effectiveness and tolerance dose of fission neutrons in canine skin for a potential combination of neutron capture therapy and fast-neutron therapy.

To investigate the potential efficacy of fission neutrons from a fast-neutron reactor for the treatment of radioresistant tumors, the relative biological effectiveness (RBE) and tolerance dose of fission neutrons in canine skin were determined. The forelimbs of 34 healthy mongrel dogs received a single dose of fission neutrons (5.6, 6.8, 8.2, 9.6 or 11 Gy) or 137Cs gamma rays (10, 15, 20, 25 or 30 Gy). Based on observations of radiodermatitis for each radiation, the single-fraction RBE of fission neutrons in the sixth month was calculated as approximately 3. The tolerance doses of fission neutrons and gamma rays, defined as the highest doses giving no moist desquamation on the irradiated skin in the recovery phase, were estimated as 7.6 Gy and 20 Gy, respectively. The tolerance dose of 7.6 Gy of fission neutrons included 5.0 Gy of fast neutrons possessing high anti-tumor effects and 1.4 x 10(12) n/cm2 of thermal neutrons, which could be applicable to neutron capture therapy (NCT). The combination of fast-neutron therapy and NCT using a fast-neutron reactor might be useful for the treatment of radioresistant tumors.

Animals↗
PubMed1994

Cell survival measurements in an argon, aluminium and sulphur filtered neutron beam: a comparison with 24 keV neutrons and relevance to boron neutron capture therapy.

Boron neutron capture therapy (BNCT) has been advanced as a suitable alternative therapy for the treatment of glioma. BNCT involves the selective uptake of a tumour with a boron-bearing substance and subsequent irradiation with a beam of neutrons. Previous attempts with BNCT have utilized thermal neutrons, but this involves resection of the scalp prior to treatment and is only possible with superficial tumours. An alternative is to use a beam of intermediate-energy neutrons which will produce a peak in the thermal neutron fluence at depth in tissue and so enable deep-seated tumours to be treated. A neutron beam with a mean energy of approximately 9 keV, obtained by filtering neutrons from a reactor with aluminium, argon and sulphur, has been used to explore the radiobiological advantage over thermal and 24 keV neutrons for BNCT. Irradiation of V79 and HeLa cells at various positions in a polythene phantom suggest that the beam is less cytotoxic for a given neutron fluence than the 24 keV neutron beam previously considered as an alternative to thermal neutrons for BNCT. However, optimization of boron distribution via the development of new compounds still appears to be necessary for BNCT to become a safe alternative option for the treatment of glioma.

Aluminum↗
PubMed1999

Boron neutron capture enhancement (BNCE) of fast neutron irradiation for glioblastoma: increase of thermal neutron flux with heavy material collimation, a theoretical evaluation.

Despite the fact that fast neutron irradiation of glioblastoma has shown on autopsies an ability to sterilize tumors, no therapeutic windows have been found for these particles due to their toxicity toward normal brain. Therefore, the Boron Neutron Capture Enhancement (BNCE) of fast neutron beam has been suggested. This paper addresses the problem of fast neutron beam collimation, which induces a dramatic decrease of the thermal neutron flux in the depth of the tissues when smaller irradiation fields are used. Thermoluminescent dosimeter TLD-600 and TLD-700 were used to determine the thermal neutron flux within a Plexiglas phantom irradiated under the Nice Biomedical Cyclotron p(60)+Be(32) fast neutron beam. A BNCE of 4.6% in physical dose was determined for a 10 x 10 cm2 field, and of 10.4% for a 20 x 20 cm2 one. A Dose Modification Factor of 1.19 was calculated for CAL 58 glioblastoma cells irradiated thanks to the larger field. In order to increase the thermal flux in depth while shaping the beam, heavy material collimation was studied with Monte Carlo simulations using coupled FLUKA and MCNP-4A codes. The use of 20 cm width lead blocks allowed a 2 fold thermal neutron flux increase in the depth of the phantom, while shielding the fast neutron beam with a fast neutron dose transmission of 23%. Using the DMF of 1.19, a BNCE of 40% was calculated in the beam axis. This enhancement might be sufficient to open, at least theoretically, a therapeutic window.

Beryllium↗
PubMed1981

Neutron spectra and neutron kerma derived from activation and fission detector measurements in a d+T neutron therapy beam.

Neutron spectra at different positions in a phantom have been derived from activation foil and fission counter measurements for the collimated beam of the Amsterdam d+T fast-neutron therapy facility. The spectra show that the fraction of lower-energy neutrons increases with increasing depth in the phantom as well as with increasing distance from the central axis of the beam. Calculation of the ratios of kerma values in tissue and tissue-equivalent (TE) plastic and of WN values for the spectra at five positions in the phantom, showed that the variations in these quantities are less than 0.5%. Kerma values have been derived from the neutron spectra and from the fission counter measurements separately. These neutron kerma values were compared with the neutron absorbed dose values derived from combined TE ionisation chamber and GM counter measurements. The dose values have been obtained applying recommendations given in the European protocol for neutron dosimetry for external beam therapy. At 50 mm and 100 mm depth along the central axis of the beam in the phantom, the agreement between the three methods was within 2%. In the penumbra regions a maximum difference of about 7% has been observed between the three methods. The contribution from thermal neutrons to the total kerma is less than 1% in the beam as well as in the penumbra.

Fast Neutrons↗
PubMed1999

Sensitivity studies of beam directionality, beam size, and neutron spectrum for a fission converter-based epithermal neutron beam for boron neutron capture therapy.

Sensitivity studies of epithermal neutron beam performance in boron neutron capture therapy are presented for realistic neutron beams with varying filter/moderator and collimator/delimiter designs to examine the relative importance of neutron beam spectrum, directionality, and size. Figures of merit for in-air and in-phantom beam performance are calculated via the Monte Carlo technique for different well-optimized designs of a fission converter-based epithermal neutron beam with head phantoms as the irradiation target. It is shown that increasing J/phi, a measure of beam directionality, does not always lead to corresponding monotonic improvements in beam performance. Due to the relatively low significance, for most configurations, of its effect on in-phantom performance and the large intensity losses required to produce beams with very high J/phi, beam directionality should not be considered an important figure of merit in epithermal neutron beam design except in terms of its consequences on patient positioning and collateral dose. Hardening the epithermal beam spectrum, while maintaining the specific fast neutron dose well below the inherent hydrogen capture dose, improves beam penetration and advantage depth and, as a desirable by-product, significantly increases beam intensity. Beam figures of merit are shown to be strongly dependent on beam size relative to target size. Beam designs with J/phi approximately 0.65-0.7, specific fast neutron doses of 2-2.6x10(-13) Gy cm2/n and beam sizes equal to or larger than the size of the head target produced the deepest useful penetration, highest therapeutic ratios, and highest intensities.

Biophysical Phenomena↗
PubMed2005

Boron neutron capture enhancement of fast neutron radiotherapy utilizing a moderated fast neutron beam.

An investigation of the therapeutic potential of boron neutron capture (BNC) enhancement of fast neutron therapy utilizing the Harper University Hospital superconducting cyclotron-produced d(48.5)+Be fast neutron therapy beam is presented. A technique for modification of the fast neutron beam to increase the BNC enhancement is presented along with an evaluation of the effects of beam moderation on the biological effectiveness of the absorbed dose. Characteristics of the photon, neutron, and boron neutron capture components of the absorbed dose are presented. Results demonstrate the possibility of therapeutic gains greater than 50% over conventional fast neutron therapy at depths required to treat brain lesions. This enhancement is estimated assuming currently achievable boron concentrations, and is more than adequate to provide a therapeutic window for the effective treatment of Glioblastoma Multiforme without prohibitive toxicity to the normal brain.

Body Burden↗
PubMed1990

The possible use of a spallation neutron source for neutron capture therapy with epithermal neutrons.

Spallation is induced in a heavy material by 72-MeV protons. The resulting neutrons can be characterized by an evaporation spectrum with a peak energy of less than 2 MeV. The neutrons are moderated in two steps: first in iron and then in carbon. Results from neutron fluence measurements in a perspex phantom placed close to the moderator are presented. Monte Carlo calculations of neutron fluence in a water phantom are also presented under some chosen configurations of spallation source and moderator. The calculations and measurements are in good agreement and show that, for proton currents of less than 0.5 mA, useful thermal-neutron fluences are attainable in the depth of the brain. However, the dose contribution from the unavoidable gamma background component has not been included in the present investigation.

Boron↗
PubMed2000

A comparison of the potential therapeutic gain of p(66)/Be neutrons and d(14)/Be neutrons.

PURPOSE: To determine the relationship between photon sensitivity and neutron sensitivity and between neutron RBE and photon resistance for two neutron modalities (with mean energies of 6 and 29 MeV) using human tumor cell lines spanning a wide range of radiosensitivities, the principal objective being whether or not a neutron advantage can be demonstrated. METHODS AND MATERIALS: Eleven human tumor cell lines with mean photon inactivation doses of 1.65-4. 35 Gy were irradiated with 0-5.0 Gy of p(66)/Be neutrons (mean energy of 29 MeV) at Faure, S.A. and the same plating was irradiated on the same day with 0-10.0 Gy of Cobalt-gamma-rays. Twelve human tumor cell lines, many of which were identical with the above selection, and spanning mean photon inactivation doses of 1.75-4.08 Gy, were irradiated with 0-4 Gy of d(14)/Be neutrons (mean energy of 6 MeV) and with 0-10 Gy of 240 kVp X-rays at the Essen Klinikum. Cell survival was determined by the clonogenic assay, and data were fitted to the linear quadratic equation. RESULTS: 1. Using the mean inactivation dose, a significant correlation was found to exist between neutron sensitivity and photon sensitivity. However, this correlation was more pronounced in the Faure beam (r(2) = 0.89, p </= 0.0001) than in the Essen beam (r(2) = 0.65, p = 0.0027). 2. No significant relationship could be established between neutron RBE and photon resistance for both modalities (p = 0.69 and p = 0.07, respectively). 3. Using alpha-coefficients as a criterion, the neutron sensitivity for the Faure beam correlated with photon sensitivity (p = 0.001), but this did not apply to the Essen beam (p = 0.27). 4. The neutron RBE for the Essen beam derived from alpha-coefficients showed a steep increase with photon resistance (p = 0.003). In the Faure beam there was no increase of RBE with photon resistance (p = 0.494). CONCLUSION: Radiobiological differences between high-energy and low-energy neutrons are particularly apparent in the dependence of the neutron RBE on photon sensitivity. The increase of RBE with photon resistance is more pronounced in the low-energy Essen neutrons than in the high-energy Faure neutrons. An RBE advantage is indicated for photon-resistant cell lines and this is particularly apparent in the low-dose range using alpha-coefficients as compared to the mean inactivation dose. The clinical application of low-energy neutrons may be more restricted because of poor penetration and lack of skin sparing. However, these neutrons discriminate better between photon-sensitive and photon-resistant cells giving an RBE range of 2-6 and a mean RBE of 4.1, than high-energy neutrons where the RBE range is 1.6-3.5 and the mean RBE is 2.4. From the radiobiological point of view it, therefore, appears that the clinical potential of low-energy neutrons is considerably underrated.

Beryllium↗
PubMed1994

Enhancement of the epithermal neutron beam used for boron neutron capture therapy.

PURPOSE: This report describes a study to enhance the epithermal neutron beam at the Brookhaven Medical Research Reactor by increasing the epithermal neutron flux and/or reducing contamination by fast neutrons. METHODS AND MATERIALS: The beam was reevaluated using Monte Carlo calculations and flux and dose measurements in air and in an ellipsoidal head phantom at the patient irradiation port. Changes in its geometry and materials were considered, including rearranging the fuel elements in the reactor core and redesigning the moderator and the patient irradiation port. RESULTS: Calculations of the new fluxes and doses at the patient irradiation port showed that the epithermal neutron flux can be increased by 100%, while the fast neutron dose per epithermal neutron can be reduced by 38%. In 1992, some of the proposed changes were made. In June 1992, measurements were made after one additional fuel element was added to replace a graphite spacer block on the epithermal beam side of the reactor core. The results show that the epithermal neutron flux increased by 18%, as predicted by the Monte Carlo calculations. In October 1992, the fuel elements in the reactor core were rearranged by placing four new fuel elements in the first row facing the epithermal port; the intensity of the epithermal neutron beam increased by 50% and the fast neutron and gamma doses per epithermal neutron may have decreased slightly. CONCLUSION: The epithermal neutron beam at the Brookhaven Medical Research Reactor has gained a 50% increase in the epithermal neutron flux and the fast neutron and gamma doses per epithermal neutron are reduced slightly after the rearrangement of the fuel elements in the core.

Boron Neutron Capture Therapy↗
PubMed1988

A review of boron neutron capture therapy (BNCT) and the design and dosimetry of a high-intensity, 24 keV, neutron beam for BNCT research.

This paper reviews the development of boron neutron capture therapy (BNCT) and describes the design and dosimetry of an intermediate energy neutron beam, developed at the Harwell Laboratory, principally for BNCT research. Boron neutron capture therapy is a technique for the treatment of gliomas (a fatal form of brain tumour). The technique involves preferentially attaching 10B atoms to tumour cells and irradiating them with thermal neutrons. The thermal neutron capture products of 10B are short range and highly damaging, so they kill the tumour cells, but healthy tissue is relatively undamaged. Early trials required extensive neurosurgery to exposure the tumour to the thermal neutrons used and were unsuccessful. It is thought that intermediate-energy neutrons will overcome many of the problems encountered in the early trials, because they have greater penetration prior to thermalization, so that surgery will not be required. An intermediate-energy neutron beam has been developed at the Harwell Laboratory for research into BNCT. Neutrons from the core of a high-flux nuclear reactor are filtered with a combination of iron, aluminium and sulphur. Dosimetry measurements have been made to determine the neutron and gamma-ray characteristics of this beam, and to monitor them throughout the four cycles used for BNCT research. The beam is of high intensity (approximately 2 x 10(7) neutrons cm-2 s-1, equivalent to a neutron kerma rate in water of 205 mGy h-1) and nearly monoenergetic (93% of the neutrons have energies approximately 24 keV, corresponding to 79% of the neutron kerma rate).

Boron↗
PubMed1996

The neutron sensitivity of dosimeters applied to boron neutron capture therapy.

To obtain a high accuracy in the dosimetry of an epithermal neutron beam used for boron neutron capture therapy (BNCT), the neutron sensitivity of dosimeters applied to determine the various dose components in-phantom has been investigated. The thermal neutron sensitivity of Mg(Ar) ionization chambers, TE(TE) ionization chambers, and thermoluminescent dosimeters (TLD) has been experimentally determined in a "pure" thermal neutron beam. Values much higher than theoretically expected were obtained and a variation up to a factor of 2.5 was found between values for the thermal neutron sensitivity of different Mg(Ar) ionization chambers of the same type. The sensitivity of the TE(TE) ionization chamber to intermediate and fast neutrons (kt) has been calculated for the neutron energy spectrum in a phantom irradiated by a clinical epithermal BNCT beam, obtained using Monte Carlo simulations. The kt value for muscle tissue ranged from 0.87 at small depths to 0.93 at larger depths in the phantom. The application of the thermal neutron sensitivities to measurements in a phantom irradiated by the epithermal BNCT beam yielded up to 17% higher gamma-ray dose rate values compared with measurements using 6Li containing caps to shield the detectors from thermal neutrons, due to a substantial perturbation of the in-phantom radiation field by the 6Li cap. The application of the new kt values resulted in a dose from intermediate and fast neutrons about 10% higher than the dose based on currently applied relative neutron sensitivities of TE(TE) chambers in BNCT beams. The resulting improvement in the accuracy of the determination of the dose from gamma rays and intermediate and fast neutrons is important in view of the required accuracy for dosimetry in radiotherapy.

Biophysical Phenomena↗
PubMedDate not supplied

Accelerator-based epithermal neutron sources for boron neutron capture therapy of brain tumors.

This paper reviews the development of low-energy light ion accelerator-based neutron sources (ABNSs) for the treatment of brain tumors through an intact scalp and skull using boron neutron capture therapy (BNCT). A major advantage of an ABNS for BNCT over reactor-based neutron sources is the potential for siting within a hospital. Consequently, light-ion accelerators that are injectors to larger machines in high-energy physics facilities are not considered. An ABNS for BNCT is composed of: (1) the accelerator hardware for producing a high current charged particle beam, (2) an appropriate neutron-producing target and target heat removal system (HRS), and (3) a moderator/reflector assembly to render the flux energy spectrum of neutrons produced in the target suitable for patient irradiation. As a consequence of the efforts of researchers throughout the world, progress has been made on the design, manufacture, and testing of these three major components. Although an ABNS facility has not yet been built that has optimally assembled these three components, the feasibility of clinically useful ABNSs has been clearly established. Both electrostatic and radio frequency linear accelerators of reasonable cost (approximately 1.5 M dollars) appear to be capable of producing charged particle beams, with combinations of accelerated particle energy (a few MeV) and beam currents (approximately 10 mA) that are suitable for a hospital-based ABNS for BNCT. The specific accelerator performance requirements depend upon the charged particle reaction by which neutrons are produced in the target and the clinical requirements for neutron field quality and intensity. The accelerator performance requirements are more demanding for beryllium than for lithium as a target. However, beryllium targets are more easily cooled. The accelerator performance requirements are also more demanding for greater neutron field quality and intensity. Target HRSs that are based on submerged-jet impingement and the use of microchannels have emerged as viable target cooling options. Neutron fields for reactor-based neutron sources provide an obvious basis of comparison for ABNS field quality. This paper compares Monte Carlo calculations of neutron field quality for an ABNS and an idealized standard reactor neutron field (ISRNF). The comparison shows that with lithium as a target, an ABNS can create a neutron field with a field quality that is significantly better (by a factor of approximately 1.2, as judged by the relative biological effectiveness (RBE)-dose that can be delivered to a tumor at a depth of 6cm) than that for the ISRNF. Also, for a beam current of 10 mA, the treatment time is calculated to be reasonable (approximately 30 min) for the boron concentrations that have been assumed.

Beryllium↗
PubMed1993

Monte Carlo calculation of dose enhancement by neutron capture of 10B in fast neutron therapy.

Since 1978 the Essen Medical Cyclotron Facility has been used for fast neutron therapy. The treatment of deep-seated tumours by d(14) + Be neutron beam therapy (mean energy = 5.8 MeV) is still limited because of the steep decrease in depth-dose distribution. The interactions of fast neutrons in tissue leads to a thermal neutron distribution. These partially thermalized neutrons can be used to produce neutron capture reactions with 10B. Thus incorporation of 10B in tumours treated with fast neutrons will increase the relative local tumour dose due to the reaction 10B (n, alpha) 7Li. The magnitude of dose enhancement by 10B depends on the distribution of the thermal neutron fluence, 10B concentration, field size of the neutron beam, beam energy and the specific phantom geometry. The slowing down of the fast neutrons, resulting in a thermal neutron distribution in a phantom, has been computed using a Monte Carlo model. This model, which includes a deep-seated tumour, was experimentally verified by measurements of the thermal neutron fluence rate in a phantom using neutron activation of gold foil. When non-boronated water phantoms were irradiated with a total dose of 1 Gy at a depth of 6 cm, the thermal fluencies at this depth were found to be 2 x 10(10) cm-2. The absorbed dose in a tumour with 100 ppm 10B, at the same depth, was enhanced by 15%.

Boron Neutron Capture Therapy↗
PubMed1994

A fundamental study on hyper-thermal neutrons for neutron capture therapy.

The utilization of hyper-thermal neutrons, which have an energy spectrum with a Maxwellian distribution at a higher temperature than room temperature (300 K), was studied in order to improve the thermal neutron flux distribution at depth in a living body for neutron capture therapy. Simulation calculations were carried out using a Monte Carlo code 'MCNP-V3' in order to investigate the characteristics of hyper-thermal neutrons, i.e. (i) depth dependence of the neutron energy spectrum, and (ii) depth distribution of the reaction rate in a water phantom for materials with 1/v neutron absorption. It is confirmed that hyper-thermal neutron irradiation can improve the thermal neutron flux distribution in the deeper areas in a living body compared with thermal neutron irradiation. When hyper-thermal neutrons with a 3000 K Maxwellian distribution are incident on a body, the reaction rates of 1/v materials such as 14N, 10B etc are about twice that observed for incident thermal neutrons at 300 K, at a depth of 5 cm. The limit of the treatable depth for tumours having 30 ppm 10B is expected to be about 1.5 cm greater by utilizing hyper-thermal neutrons at 3000 K compared with the incidence of thermal neutrons at 300 K.

Brain↗
PubMed1998

Combined use of FLUKA and MCNP-4A for the Monte Carlo simulation of the dosimetry of 10B neutron capture enhancement of fast neutron irradiations.

Boron neutron capture enhancement (BNCE) of the fast neutron irradiations use thermal neutrons produced in depth of the tissues to generate neutron capture reactions on 10B within tumor cells. The dose enhancement is correlated to the 10B concentration and to thermal neutron flux measured in the depth of the tissues, and in this paper we demonstrate the feasibility of Monte Carlo simulation to study the dosimetry of BNCE. The charged particle FLUKA code has been used to calculate the primary neutron yield from the beryllium target, while MCNP-4A has been used for the transport of these neutrons in the geometry of the Biomedical Cyclotron of Nice. The fast neutron spectrum and dose deposition, the thermal flux and thermal neutron spectrum in depth of a Plexiglas phantom has been calculated. The thermal neutron flux has been compared with experimental results determined with calibrated thermoluminescent dosimeters (TLD-600 and TLD-700, respectively, doped with 6Li or 7Li). The theoretical results were in good agreement with the experimental results: the thermal neutron flux was calculated at 10.3 X 10(6) n/cm2 s1 and measured at 9.42 X 10(6) n/cm2 s1 at 4 cm depth of the phantom and with a 10 cm X 10 cm irradiation field. For fast neutron dose deposition the calculated and experimental curves have the same slope but different shape: only the experimental curve shows a maximum at 2.27 cm depth corresponding to the build-up. The difference is due to the Monte Carlo simulation which does not follow the secondary particles. Finally, a dose enhancement of, respectively, 4.6% and 10.4% are found for 10 cm X 10 cm or 20 cm X 20 cm fields, provided that 100 micrograms/g of 10B is loaded in the tissues. It is anticipated that this calculation method may be used to improve BNCE of fast neutron irradiations through collimation modifications.

Biophysical Phenomena↗