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ORIGINAL ARTICLE
Year : 2017  |  Volume : 13  |  Issue : 3  |  Page : 456-465

Accelerator driven neutron source design via beryllium target and 208Pb moderator for boron neutron capture therapy in alternative treatment strategy by Monte Carlo method


Cellular and Molecular Gerash Research Center, Gerash School of Paramedical Sciences, Shiraz University of Medical Sciences, Shiraz, Iran

Date of Web Publication31-Aug-2017

Correspondence Address:
Abdollah Khorshidi
Cellular and Molecular Gerash Research Center, Gerash School of Paramedical Sciences, Shiraz University of Medical Sciences, P. O. Box: 7441758666, Shiraz
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.179180

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 > Abstract 

Aims: The reactor has increased its area of application into medicine especially boron neutron capture therapy (BNCT); however, accelerator-driven neutron sources can be used for therapy purposes. The present study aimed to discuss an alternative method in BNCT functions by a small cyclotron with low current protons based on Karaj cyclotron in Iran.
Materials and Methods: An epithermal neutron spectrum generator was simulated with 30 MeV proton energy for BNCT purposes. A low current of 300 μA of the proton beam in spallation target concept via 9Be target was accomplished to model neutron spectrum using 208Pb moderator around the target. The graphite reflector and dual layer collimator were planned to prevent and collimate the neutrons produced from proton interactions. Neutron yield per proton, energy distribution, flux, and dose components in the simulated head phantom were estimated by MCNPX code.
Results: The neutron beam quality was investigated by diverse filters thicknesses. The maximum epithermal flux transpired using Fluental, Fe, Li, and Bi filters with thicknesses of 7.4, 3, 0.5, and 4 cm, respectively; as well as the epithermal to thermal neutron flux ratio was 161. Results demonstrated that the induced neutrons from a low energy and low current proton may be effective in tumor therapy using 208Pb moderator with average lethargy and also graphite reflector with low absorption cross section to keep the generated neutrons.
Conclusions: Combination of spallation-based BNCT and proton therapy can be especially effective, if a high beam intensity cyclotron becomes available.

Keywords: Boron neutron capture therapy, effective radiation dose, epithermal neutrons, small cyclotrons, spallation target concept


How to cite this article:
Khorshidi A. Accelerator driven neutron source design via beryllium target and 208Pb moderator for boron neutron capture therapy in alternative treatment strategy by Monte Carlo method. J Can Res Ther 2017;13:456-65

How to cite this URL:
Khorshidi A. Accelerator driven neutron source design via beryllium target and 208Pb moderator for boron neutron capture therapy in alternative treatment strategy by Monte Carlo method. J Can Res Ther [serial online] 2017 [cited 2020 May 27];13:456-65. Available from: http://www.cancerjournal.net/text.asp?2017/13/3/456/179180


 > Introduction Top


Malignant brain tumors are called glioblastoma multiforme, and other tumors as anaplastic astrocytoma cannot be effectively treated by conventional therapy via surgery, chemotherapy, and radiation therapy; hence, boron neutron capture therapy (BNCT) was born to aim at these tricky cancers. BNCT is a technique which is employed to selectively target high linear energy transfer heavy charged particle radiation to tumors at the cellular level. When 10B absorbs a thermal neutron the energetic emitted alpha particle and recoil 7Li ion produced via (n, α) reaction deposits energy locally averaging about 2.33 MeV. About 94% of all the reaction leads to the recoiling of 7Li ion produced in an excited state and de-excite by emitting a 477 keV gamma ray. In the remaining events, the 7Li is emitted in the ground state with no gamma-ray emission, which can be ignored from a dosimetry perspective, although they are frequently utilized for Boron-10 analysis purposes.[1],[2],[3] Delivery of 10B agents to brain tumors depends on (a) low systemic toxicity and normal tissue uptake with high tumor uptake and concomitantly high tumor/brain and tumor/blood concentration ratios (>3–4:1), (b) tumor concentrations of ~20 μg 10B/g tumor, (c) rapid clearance from blood and normal tissues and persistence in tumor during BNCT, (d) chemical stability, and (e) water solubility.

Thermal neutrons (0–1 eV) have a limited depth of penetration, then the epithermal neutrons (1 eV to 10 keV), which lose energy and fall into the thermal range as they penetrate tissues, are now preferred for clinical therapy. In recent years, the reactor has increased its area of application into medicine, in particular radioisotope production, as well as BNCT.[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22] Accelerators also can be used to produce neutron sources;[23],[24],[25],[26],[27],[28],[29] accelerator-based methods are compact enough to be sited in hospitals, thereby allowing for more effective but technically more complicated procedures to carry out BNCT. Montagnini et al.[30] have investigated neutron spectrum shaping based on the conventional accelerator with 2.5 MeV proton energy and 10 mA current. Meanwhile, Tanaka et al.[31] have settled the method of moderating fast neutrons which were emitted from the reaction between the beryllium target and 30 MeV protons accelerated by a cyclotron. In this research, a new design of the neutron activator using the lower energy of incident protons with a lower current beam is investigated for BNCT applications.

For both reactor and accelerator-based neutron sources, a moderator assembly is necessary to reduce the energy of the neutrons to the epithermal range. The generated neutron source has a distribution of energies and is accompanied by unwanted X-rays and γ photons. A fundamental principle of BNCT is that the dose of neutrons delivered to the target volume should not exceed the tolerance of normal tissues, and this applies to neutron beam design as well as to treatment planning.[32] The implication of this for beam design is that the negative consequences of increased normal tissue damage for more energetic neutron beams at shallow depths outweigh the benefits of more deeply penetrating energetic neutrons.

The safeties standards for the development of the BNCT neutron generator are in compliance with the technical document (International Atomic Energy Agency [IAEA]-TECDOC-1223) have been compiled in 2001 by the IAEA.[33] The recommended flux for epithermal neutrons is 1E+9 n/cm2/s and also fast neutron and gamma ray components should be 2E−13 Gy.cm2 per epithermal neutron, which have the most effectiveness on deep-seated tumors. Designing of beam shaping assembly structure via accelerator-based method and distinctive moderator material is needed to generate an appropriate neutron beam. Moreover, Fermi's age theory has been consisted of moderating neutrons by scattering with relatively little energy absorption. 208Pb has the lowest capture cross section for the fast neutron, and also neutrons have a small average lethargy due to the high atomic mass of lead. In this work according to the Karaj cyclotron (Cyclon 30, IBA, Belgium, and 300 μA proton beam current) in Iran, the design of the whole facility was simulated and modeled by spallation target concept via planned neutron activator. The induced neutron source from the reaction in the 208Pb moderator region amassed via graphite reflector as arc shape around the moderator, then the neutrons guided and modified by collimator and filters toward head phantom. Generation of neutron to proton ratio from beryllium target, energy distribution, flux of induced neutrons in the region of 208Pb moderator, and dose evaluation in tumor and brain phantom were simulated by MCNPX code.[34]


 > Materials and Methods Top


Target design

Spallation target concept is usually the bombardment of a metal target by an intense beam of incident particles so as to produce high-intensity fluxes of neutrons. Here, the high power spallation source idea will be utilized through low energy and low current protons in the prediction of neutron production for treatment purposes.

To simulate the system a current of 300 μA of the proton beam with a Gaussian based on Karaj cyclotron, five proton sources with energies of 30, 28, 26, 20, and 15 MeV incidents on beryllium target to produce neutrons were considered. The beryllium was considered as the spallation target material because of its high melting point (1287°C) and reasonably low activation level. The target was designed as a cylindrical shape with a 0.83 cm radius and 2 cm length.

The deposited energy in the beryllium target depends on total stopping power, current, and energy of protons. The low energy proton with 30 MeV at 1 μA current is lost 28.5 watts per cm in the beryllium target as i(dE/dx)total, where i is the number of incident protons per se cond, and (dE/dx)total is total stopping power (MeV/cm).

The low energy proton in 30 and 15 MeV at 300 μA current is produced 8550 and 15,100 joule/s heat per centimeter in the beryllium target, respectively. The loss of power per unit length increased when the energy of proton decreased for different proton energies. Furthermore, higher energies lead to more heat transfer. The simulated target was equipped with a cooling system, and water was employed as its coolant. The ANSYS software (ANSYS Inc., USA)[35] was used to estimate the target temperature and the cooling water velocity via computational fluid dynamics (CFD) method. Simulations of the whole therapeutic facility were done via the MCNPX code[34] where the microscopic cross sections according to ENDF/B-VII.1 library were taken into account.[36]

Moderation and reflection procedures

Generated neutrons from spallation target are spread out around the target. The produced fast neutrons are slowed down via the moderation process in spherical geometry of 208Pb around the target. The 208Pb with an elastic cross section of 11.46 barn between 100 keV and 0.5 eV neutron energy was selected as neutron moderator because of a low scattering cross section at desirable epithermal energies, high scattering cross section at higher energies, high elastic collisions, and 52.40% abundance.[37] The reduced momentums of neutrons toward the epithermal range of energy are resulted from elastic collisions between induced neutrons and 208Pb nuclides. The 208Pb moderator was simulated as spherical geometry with a 30 cm radius to estimate neutron flux.

The induced neutrons from the target in the activator region needed to be decelerated in the lead region and then guided to the beam port with a reflector material. The schematic of neutron activator and storage space of neutrons has been shown in [Figure 1]. The reflector was designed in an arc shape around the lead buffer in order to prevent neutron loss. A suitable reflector material is supposed to have a high elastic scattering cross section and a low absorption cross section to allow neutrons re-entry to the moderator region. In the design, graphite with a thickness of 25 cm is proposed as a reflector to preserve the neutrons in a superior flux.
Figure 1: The plan of the projected design in boron neutron capture therapy, and the thicknesses are as follow: 1 = 0.83 cm in radius and 2 cm in length beryllium, 2 = 30 cm in radius 208Pb moderator, 3 = 25 cm in thickness graphite reflector, 4 = Fluental moderator, 5 = 1st Fe filter, 6 = 0.5 cm Li filter, 7 = Bi filter, 8 = 2nd Fe filter, 9 = B4C frustum 65 cm gateway radius and 4 cm end of cone radius, 10 = Bi frustum 68 cm entrance radius and 5 cm egress radius, 11 = 0.2 cm scalp, 12 = 2 cm skull, 13 = 10 cm in radius of brain, 14 = 1 cm in radius of tumor at depth of 1 cm inside brain region, 15 = Lead shield

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Collimator and filter outlines

Guiding the appropriate beam components for epithermal neutrons in the direction of the phantom necessitates a collimator and filter assemblies. The collimator is made up of two layers of boron carbide (B4C) and Bi materials. The internal layer of the collimator acts as an absorber which should eliminate the dispersed neutrons. The external layer works as a gamma shield, which is supposed to reduce hard gamma rays. The filtered beam facilities support the proper flux of epithermal neutrons within air with the order of 1E+9 n/cm2/s, and likewise IAEA-TECDOC-1223, 2001[33] and Kiger 3rdet al. 1999[38] have recommended beam quality parameters and corresponding neutron beam energy limits in BNCT. The epithermal energy group is between 1 eV and 10 keV. In our design, the surface and volumetric flux have been simulated by desired scoring tools, as the number of histories was 108. The unit of a surface or volume and time normalized output is particle/cm2/s. When the code was run by the defined protons, the protons tracked it through the simulated activator and particle history weights were binned into the energy spectra.

Head phantom model

The scalp-skull-brain phantom consisting of homogeneous regions of bone or brain equivalent material was simulated in order to assess the dosimetric effect in the brain. The elemental composition of the Monte Carlo simulations according to ICRU report 46 (ICRU 1992) was used. The geometry of the phantom was defined as a spherical shape, and the 1 cm–seated tumor in the head phantom was compounded at 7.14% 1H, 57.14% 16O, and 35.72%10B concentration which the density of this segment was calculated 1.77 g/cm3.

Epithermal neutrons can pass through the scalp, temporal muscle, and the cranial bone and convert to thermal neutrons in tissue. Therefore, epithermal neutrons would improve the amount of thermal neutrons delivered to deep-seated lesions. Subsequently, it was determined that for deep-seated brain tumors, a beam of epithermal neutrons, defined as neutrons with energies between 1 eV and 10 keV, was preferable to a beam of thermal neutrons.[33]


 > Results Top


Neutron yield prediction

A conceptual design of a neutron source was proposed for low current protons via 9Be as a thick target. A parametric CFD examination of the target configuration was used to determine the cooling conditions, namely single-phase cooling flow, limited flow velocities, and smooth temperature gradients in the target body. Using a 30 MeV, 300 μA proton beam, a water flow rate of 0.90 l/s, and an inlet water temperature of 13°C, the maximum calculated temperature in the target was 489°C.

[Table 1] indicates the neutron yield for incident proton with an energy of 15–30 MeV and current of 300 μA. The beam axis was perpendicular to the base of the cylindrical target. When the energy of incident proton decreased, the amounts of neutron yield and neutron per ratio decreased. Simulation results showed that 30 MeV proton energy induced the maximum values of neutron yield and neutron per ratio.
Table 1: Neutron/proton ratio and neutron yield at 300 μA of proton by diverse energies from 9Be target

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Neutron flux analysis

The produced neutrons from 9Be target were spread inside the 208Pb region. The whole target and water cooling system were integrated into the moderator/reflector assembly without adversely affecting the neutron field either by reducing the flux or by altering the desired neutron energy spectrum. [Table 2] displays the flux of gathered neutrons in the moderator and reflector regions for diverse energies of protons. The flux of collected neutrons was divided into four energy groups. Moreover, while the proton energy decreased, the volumetric flux of neutrons reduced inside both the moderator and reflector regions.
Table 2: Neutron flux (n/cm2/s) in the moderator and reflector regions from proton interaction with beryllium target in different energy groups

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[Table 3] demonstrates the surface flux of derived neutrons by score surface over the mesh output surfaces at the end of the collimator (in air) from thickness changing in Fluental and the first Fe filters while the proton energy on target is 30 MeV. The thicknesses of Fluental and 1st Fe filters were considered between 1.4–5.4 cm and 3–7 cm, respectively, and 2nd Fe, Bi, and Li filters were kept constant at 1, 5, and 0.5 cm, respectively. The minimum flux of fast and thermal neutrons occurred in 5.4 cm Fluental and 3 cm 1st Fe. The greatest amount of epithermal neutron flux occurred at 1.4 cm Fluental and 7 cm 1st Fe. At the same time, the fast and thermal fluxes were 3.81E+8 and 1.26E+7 n/cm2/s, respectively. [Table 4] shows the surface neutron flux in different thicknesses of 2nd Fe and Bi filters at the end of the collimator (in air) with 30 MeV of proton energy, when the thicknesses of Fluental, 1st Fe, and Li filters were kept constant at 2.4, 6, and 0.5 cm, respectively.
Table 3: Surface neutron flux (n/cm2/s) in different thicknesses of Fluental and 1st Fe at the end of the collimator with 30 MeV of proton energy, when the thicknesses of 2nd Fe, Bi, and Li filters were 1 cm, 5 cm, and 0.5 cm, respectively

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Table 4: Surface neutron flux (n/cm2/s) in different thicknesses of 2nd Fe and Bi at the end of the collimator with 30 MeV of proton energy, when the thicknesses of Fluental, 1st Fe, and Li filters were 2.4 cm, 6 cm, and 0.5 cm, respectively

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In BNCT, the epithermal to fast flux ratio and the epithermal to thermal flux ratio must be larger than 20 and 100, respectively;[33] consequently we have considered 2.4 cm Fluental, 6 cm 1st Fe, 0.5 cm Li, 4.8 cm Bi, and 1.2 cm 2nd Fe to evaluate other proton energies in this proposed scheme, as well as [Table 5] displays the surface neutron flux at the end of the collimator for diverse energies of the incident proton. While proton energy decreased, the thermal neutron flux reduced for all proton energies. [Table 6] demonstrates the flux at the end of the collimator with 2.4 cm Fluental, 6 cm 1st Fe, 0.5 cm Li, and 6 cm Bi without 2nd Fe filter, as well as the incident neutron flux on the phantom fluctuated because of each filter performance with distinct thicknesses. Moreover, the maximum flux of epithermal neutrons occurred in 20 MeV; at the same time, the fast and thermal fluxes were 1.16E+8 and 2.21E+7 n/cm2/s, respectively. The comparison of [Table 5] and [Table 6] indicates that the lower proton energies of 26 and 20 MeV give the best results in epithermal to fast and thermal ratios according to the IAEA-TECDOC-1223 recommendations. The Fe filter has high inelastic scattering cross section above 860 keV as well as it decreases the very fast neutron flux over the range of 1 MeV. Meanwhile, [Table 7] exhibits the surface neutron flux for 30 MeV energy of incident proton with different thicknesses of filters exclusive of 2nd Fe filter. When the Bi thickness was 4 cm, using 7.4 cm Fluental and 3 cm Fe revealed the maximum epithermal flux, as well as the epithermal to thermal neutron flux ratio was 161. Meanwhile, when the Bi thickness was kept constant in 3 cm, the maximum epithermal flux took place with a thickness of 5.4 cm Fluental and 6 cm Fe.
Table 5: Surface neutron flux (n/cm2/s) in different ranges of neutron energies at the end of the collimator for diverse energies of incident proton on beryllium target and with filter thicknesses: 2.4 cm Fluental, 6 cm 1st Fe, 0.5 cm Li, 4.8 cm Bi, and 1.2 cm 2nd Fe

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Table 6: Surface neutron flux (n/cm2/s) in different ranges of neutron energies at the end of the collimator for diverse energies of incident proton on beryllium target and with filter thicknesses: 2.4 cm Fluental, 6 cm 1st Fe, 0.5 cm Li, and 6 cm Bi

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Table 7: Surface neutron flux (n/cm2/s) in different ranges of neutron energies at the end of the collimator for 30 MeV of incident proton on beryllium target without 2nd Fe filter

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[Table 8] demonstrates the comparison of Cerullo et al.[39],[40] and Montagnini et al.[30] for an epithermal flux in the beam shaping configuration. In our work, the epithermal flux was considered for 30 MeV energy of incident proton with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi according to [Table 7]. [Table 8] indicates that in our proposed work, the epithermal to fast neutron flux ratio in comparison with Cerullo et al. 2002 work had the higher amount. In addition, the epithermal to thermal neutron flux ratio was higher than Montagnini et al. and Cerullo et al. in best configuration and also the IAEA recommended. In that case, to gain the IAEA recommended level, we had to increase the epithermal flux against the fast flux using different filters and diverse thicknesses.
Table 8: Comparison of neutron beam in this work (for Ep=30 MeV) with filter thicknesses: 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi and some published works

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The volumetric induced neutron fluxes (n/cm2/s) at the thermal, epithermal, and fast ranges have been compared in [Figure 2] inside the lead and graphite regions, and also inside air before phantom with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi filters for 30 MeV energy of incident proton. Neutron flux had varied continuous reduction rate inside different regions of the neutron activator. [Figure 3] shows a comparison of the surface induced neutron fluxes at thermal and epithermal ranges inside the phantom. It is observed that the neutron flux inside the tumor was a little larger than the flux inside the brain at the thermal range. Neutron flux had around continuous reduction rate inside different regions of the brain phantom.
Figure 2: Induced neutron flux with 30 MeV proton energy inside the lead (solid line), graphite (dash line-top), and end the collimator (dash line-bottom) between 0 and 1 MeV with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi filters

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Figure 3: Induced neutron flux with 30 MeV proton energy in air on the phantom surface (dash line-top), in skull (dash line-middle), in brain (dash line-bottom), and in tumor (solid line) with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi filters

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The surface flux of derived neutrons as beam shaping and gamma doses can be estimated in the scalp surface and inside the phantom. In the scalp surface, assessments are most useful for a general characterization of epithermal neutron beams, whereas in the phantom, estimations are necessary for providing data to compare with computational treatment planning codes. [Table 9] indicates the neutron flux and dose in the epithermal range as well as the thicknesses of Fluental, Fe, Li, and Bi filters which were 7.4 cm, 3 cm, 0.5 cm, and 4 cm, respectively, for various energies of the incident proton. It would have been supposed that the tumor depth inside the brain region is 1 cm in the direction of the axis beam.
Table 9: Simulated neutron flux and dose component in epithermal range at 300 μA of current proton with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi filters

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When the proton energy decreased, surface flux for the tumor region became reduced in all the energy ranges, and the epithermal flux of neutrons was in the order of 1E+9 n/cm2/s, with the exception of 20 and 15 MeV energies of proton. Neutron and photon dose at epithermal range in three regions of the phantom were tabulated in terms of (Gy.cm2)/#, where # is the number of neutron or photon per projectile. The value of quality factor for neutron radiation is energy-dependent, and its value in the epithermal range is 5. Photon dose increased in the scalp, brain, and tumor regions correspondingly for 30 MeV energy of the proton. When the proton energy decreased from 30 to 28 MeV, the thermal flux reduced in the scalp from 7.08E+9 to 5.15E+9 n/cm2/s because of the thickness of the constant thermal neutron absorber, so the Li thicknesses must decrease access to a higher ϕepithermal ratio. Meanwhile, in the lower energy of incident protons, fast neutron fluxes were required to improve ϕepifast ratio and gamma ray dose. Furthermore, the contamination of absorbed dose for gamma ray and fast neutrons per epithermal neutron in air at the end of the collimator were simulated 2.5 E−14 and 1.9E−13 (Gy.cm2)/n correspondingly for 30 MeV protons.

[Figure 4] shows the effective radiation dose (H) versus depth inside the simulated phantom at the epithermal range for 30 MeV proton energy, as well as changing in material density altered the dose value and also radiation biological effectiveness. This neutron spectrum revealed a delivered maximum dose in brain region inside the tumor due to the high concentration of 10B and also was proper for treatment applications.
Figure 4: Effective radiation dose versus depth in the phantom for 30 MeV proton energy with 7.4 cm Fluental, 3 cm Fe, 0.5 cm Li, and 4 cm Bi filters. The tumor has been located at 1 cm inside the brain region

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Cross sections features

The total flux of accumulated neutrons in diverse regions relies on the velocity of neutrons. Furthermore, the flux of collected neutrons is different inside the lead or graphite volume. Absorption and elastic cross sections inside 208Pb region were compared in [Figure 5], and the elastic cross section in the whole neutron energy was higher than the absorption cross section. Therefore, lead was chosen as the moderating material because of its greater elastic characteristics than other neutron moderators such as 1H, 2D, 12C, and 16O. [Figure 6] indicates the elastic cross section in the 208Pb moderator and graphite reflector, in addition to graphite, which showed less significant elastic properties than 208Pb.
Figure 5: Absorption (solid line) and elastic (dash line) cross sections of neutrons in 208Pb region

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Figure 6: Elastic cross section of neutrons in 208Pb moderator (solid line) and graphite reflector (dash line)

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10B has a very large capture cross section (3830 barns) for thermal neutrons and decays into an alpha particle and a lithium nucleus, the combined ranges of which are ~10 μm, approximately one cell in diameter. When the phantom is designed with an epithermal neutron beam, the neutrons thermalized in the tissue may be captured by 10B damaging the cells in which the capture took place. The success of this therapy depends on two factors: the selectivity of the 10B carrying drug and the availability of a neutron beam with a suitable energy spectrum and sufficient intensity. [Figure 7] shows (n, α) cross section inside the brain and tumor regions, as well as (n, α) cross section in the tumor region had higher values than the region inside the brain. In addition, in the lower energy of neutrons, the neutron capture had highly developed cross section values.
Figure 7: (n, α) cross section in brain (solid line) and in tumor (dash line) regions

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[Figure 8] shows (n, γ) cross sections in the bismuth filter and 208Pb region. The Bi had first resonance in 0.0008 MeV where it occurred at lower energy than lead resonance at 0.06 MeV. Meanwhile, the Bi had higher (n, γ) cross section values in the lower neutrons energies than 208Pb, and this property selected the Bi as the gamma filter.
Figure 8: (n, gamma) cross section in Bi material (solid line) and in 208Pb buffer (dash line)

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 > Discussion Top


The suggestion of the reactor-based method for neutron beam design is that the noncooperative outcomes of increased normal tissue damage for more energetic neutron at superficial depths be more important than the profits of more deeply penetrating fast neutrons. For fission reactors, the average energy of the neutrons produced is ~2 MeV, but small numbers have energies as high as 10 MeV. There is generally a tradeoff between treatment time and the optimum beam for patient treatment in terms of the energy distribution of the neutrons and the contamination of the neutron beam with X-rays and γ photons. Not unexpectedly, reactors with the shortest treatment time (i.e., the highest normal tissue dose rate) operate at the highest power, because the number of neutrons that is produced per unit time is proportional to the power measured in MW. Furthermore, high beam quality is most easily achieved using reactors with high power because a larger fraction of the neutrons can be filtered as the neutrons traverse the moderator assembly without making the treatment time exceedingly long.

In order to generate epithermal and thermal neutrons, we proposed a new scheme of moderating fast neutrons, which are emitted from the thick target and the low energy proton accelerated by a cyclotron in low current. In this study, using 208Pb moderator and graphite reflector, the feasibility of BNCT was investigated. Proposed method by Tanaka et al.[31],[41] and Imoto et al.[42] were based on the direct moderating fast neutrons via high current proton in 1 mA without any reflector assembly, but in our research, the neutron storage using lower current proton and graphite reflector structure was examined for BNCT applications. Induced neutrons from proton interaction were at the fast range inside the lead region at first; then the neutron fluxes decreased in the collimator region as well as for diverse proton energies recommend different thicknesses of filters and changing in Li filter thickness to gain desired epithermal flux. Neutron flux, neutron, and photon dose had approximately continuous sequence inside the scalp, tumor, and brain regions. The moderating method was explored via Fermi's age theory for the slowing down of neutrons from fast to thermal energy through collisions with the moderator's nuclei. When the epithermal neutron flux inside air before phantom is in order of 1E+9 n/cm2/s, BNCT using graphite reflector will be very attainable in spallation-based neutron source via microcurrent protons.

Meanwhile, Yonai et al.[43] have experimented cyclotron-based neutron source using 50 MeV protons and weighty target for BNCT, which epithermal neutron energy spectrum passing through moderators. But here using lower proton energy and light target, the simulated results indicated the therapy beam exit is sufficient for BNCT applications. And also, the neutron fluence rate was intensified and reserved by means of a graphite reflector placed around the lead moderator to maximize the delivery of therapeutic neutrons.

Most importantly, the Fluental moderator has the low-scattering and high absorption cross sections including some resonances at high energies which give a good moderation of neutrons down to the epithermal energies, where the cross section is much smaller. A disadvantage of the Fluental moderator, compared to other moderating materials, is readily activated by neutrons followed by emission of high energy gammas. All moderating materials can be compared using a figure of merit called the moderation ratio that depends on lethargy decrement, average energy lost by a neutron in a collision with the nuclide. Large quantity of the lethargy is better because it means that the neutron is thermalized in fewer collisions during the moderation process. The Fluental generates many more high energy photons than the graphite and lead materials when interacted by neutrons, leading to higher photon dose to a healthy tissue. Lead has a lower moderating power than graphite; therefore, if the lead would be chosen as a reflector, more fast neutrons are reflected from the lead than from the graphite reflector. By itself and by photon induction in the tissues, this higher fast neutron component in the neutron beam provides an additional dose to a healthy tissue.

Furthermore, Bleuel et al. 1999[44] have examined an accelerator-driven neutron source for spectral tailoring in BNCT using 2.5 MeV and 20 mA protons on a lithium target through the lead reflector. Nevertheless, in our study, the lead was chosen as the moderator material because of its greater elastic characteristics of absorption by way of microcurrent of incident proton. Lead has the lowest capture cross section for the fast neutron with small energy degradation steps.

The half-value layer of lead shield for 2 MeV photons is 1.3 cm by the way of –ln(0.5)/(μρ), where μ is the mass attenuation coefficient of lead for 2 MeV photons (4.6E−2 cm2/g) and ρ is the density of lead (11.35 g/cm3). Consequently, the lead shield was located in front of the head phantom to reduce the gamma dose rate. Meanwhile, the neutron energy varies at greater depths in the phantom, as well as an irradiation time on the order of several hours is needed to deliver the required dose.

In this research, various thicknesses of filters were simulated, and the individual fluxes were investigated via moderating neutrons by scattering with relatively little energy absorption in lead. In order to the reduction of the fast neutron contamination, the Fe filter performance was examined as it has high inelastic scattering cross section and decreases the very fast neutron flux. For future work, the moderator/filter design will be optimized using diverse thicknesses of moderator/reflector, different greater depth in-phantom treatment figures of merit, and likewise the beam quality.


 > Conclusions Top


The design of the whole facility was simulated by thick target using strengthening the neutron source via lead moderator and graphite reflector in an accelerator-based neutron source. The small cyclotron is of a physical size to be useful in hospitals for BNCT via the suitability of 20–30 MeV protons on 9Be neutron source. The spallation-based BNCT is feasible using microcurrent protons by adjusting the moderator/reflector/collimator's thicknesses. However, a combination of BNCT and proton therapy using two switching magnets can be especially effective if a high-beam intensity accelerator becomes available. Here, safer way suggested for BNCT purposes using small cyclotrons in low current and energy of protons as compared to the reactor-based method. By using the proposed assembly, the spallation-based neutron source can provide better dose distribution at deeper positions within a phantom than that of the presently employed reactor-based neutron sources. Presented method can be used for dual aims simultaneously, so it will give flexibility to work on radionuclide production and BNCT with minimal investment of time and staff.

Acknowledgments

Here is our thankfully acknowledge the spiritual and financial maintenance from Gerashian peoples who have constructed and supplied the Gerash University of Medical and Paramedical Sciences, Amir-Al-Momenin Hospital, Cancer Prevention Center, MRI Department, Medical and Nano Laboratories, Home residence and requirements in Fars province by Gerashian charitable peoples, particularly Sheikh-Ahmad Ansari/Mir-Abolhasan Sa'adat/Haj-Moshtaq Moshtaqi/Haj-Mohammad Barazandeh/Haj-Ahmad Mohebbi/Haj-Hossein Hosseinzadeh/Haj-Qolamhossein A'bedi/Haj-Masoud Veqarfard and also Gerash charity institution and social support with low governmental provision rate in regional public welfare and health promotion.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
 > References Top

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    Figures

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