Year 6, Number 22, October 2003

Neutron Source Design for Boron Neutron Capture Synovectomy.

Article N° AJ22- 8

Héctor René Vega-Carrillo[1,2,3]and Eduardo Manzanares-Acuña;[1,3]
Estudios Nucleares[1]; Ingeniería Eléctrica[2] y Matemáticas[3]

Correspondence:

Hector Rene Vega-Carrillo.
Universidad Autónoma de Zacatecas Cuerpo Académico de Radiobiología
Apdo. Postal 336, 98000 Zacatecas, Zac. México
E-mail: rvega@cantera.reduaz.mx
URL: http://cren.reduaz.mx/~rvega

Cita/Reference:
Vega-Carrillo,Héctor René. et al. Neutron Source Design for Boron Neutron Capture Synovectomy. Alasbimn Journal 6(22): October 2003. Article N° AJ22- 8.

 

 

 

Abstract


A neutron source for Boron Neutron Capture Synovectomy has been designed by Monte Carlo calculations. The source is based in the utilization of a 239PuBe isotopic neutron source that is located in the center of a cylindrical container filled heterogeneously with two moderators. Studied moderators were light water/heavy water, graphite/heavy water, lucite/heavy water and polyethylene/heavy water. Features such as total and thermal neutron fluence were utilized to select the best moderator array. Neutron spectrum produced by the 239PuBe inserted at polyethylene/heavy water moderator was used to determine the neutron spectra inside a knee model. During knee modeling the elemental composition of synovium and synovial fluid was assumed alike water, blood tissue and blood tissue with two different concentrations of boron. For synovium modeled as blood tissue with 0.278% in weigth of boron the neutron kerma is 7351 times larger than synovium modeled as water.

Keywords: Neutron capture, Synovectomy, 239PuBe, Monte Carlo, Moderator

 

Resumen


Se diseñó una fuente de neutrones para la Sinovectomía por Captura de Neutrones en Boro. La fuente utiliza una fuente isotópica de 239PuBe que se inserta en el centro de contenedor cilindrico con un arreglo heterogéneo de dos moderadores. Los moderadores estudiados son agua ligera/agua pesada, grafito/agua pesada, lucita/agua pesada y polietileno/agua pesada. La fluencia total y la de los neutrones térmicos producidas se emplearon para seleccionar el mejor embalaje. Así, el espectro que produce una fuente de 239PuBe dentro del moderador con polietileno/agua pesada se utilizó como término fuente para calcular los espectros de neutrones dentro de un modelo de rodilla. La composición elemental utilizada para modelar el saco y el líquido sinovial fue la del agua, la del tejido sanguíneo y la de este último con dos concentraciones de boro. Encontramos que cuando la composición elemental del saco sinovial se asume igual a la del tejido sanguíneo con 0.278% en peso de boro el kerma debido a los neutrones es 7351 veces mayor al que se obtiene cuando la composición del saco sinovial es la del agua.


Palabras clave: Captura de neutrones, Sinovectomía, 239PuBe, Monte Carlo, Moderador

 

 

 

Introduction


Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when the stable isotope 10B is irradiated with thermalized neutrons to yield alpha particles and 7Li nuclei. These are short-range high linear-energy-transfer particles with large ionization track density, which in BNCT are directed mainly against tumors. If the (n,α) nuclear reactions occurs inside the tumor cell an energy of 2.3 MeV is deposited within a circle of 10 to 12 mm (about 1 cell diameter), killing the tumoric cell. [1, 2]

For deep-seated tumor epithermal neutrons, from 1 eV to 10 keV, thermalize at a depth of about 2.5 cm, providing a maximum thermal neutron fluence at the tumor site with a minimum of damage to normal tissue. [3]

The absorbed dose in tumor and normal cell is the summation of the all-individual absorbed dose resulting from all possible nuclear reactions. The dose contribution comes from 14N(n,p)14C, 10B(n,α)7Li, proton recoil, 1H(n,γ)2D, and all (n,γ) reactions [4].

About 3% of the US population are affected by rheumatoid arthritis (RA) which manifest mainly in the synovium. Because of the pain and debilitation, patients with this degenerative disease are grateful even for symptomatic relief without a permanent cure [5].

Rheumatoid Arthritis is an autoimmune disease characterized by swollen, inflamed, and painful joints. The physical and economic disability that results is enormous, exceeded only by cardiovascular disease, arthritis disorders are the second leading cause of time and earning loss in the US. [6]

Radiation synovectomy has been pursued as an effective alternative to chemical and surgical synovectomy for arthritis treatment. Radiation synovectomy involves intraarticular injection at the synovial joint of colloids or particles incorporating a b- emiting radionuclide such as 165Dy, 199Au, 32P, 153Sm, 166Ho, 186/188Re and 90Y. The inflamed synovium is ablated and the mechanism of action is believed to be the engulfment of particles by macrophages present in large numbers, which then get irradiated and destroyed [5,6].

Some drawbacks of radiation synovectomy are the probable transport of radionuclides to other parts of patient body and radionuclide leakage from the joint, which can produce radioactive contamination and radiation exposure [7, 8].

Using the same physics principles used in BNCT has been proposed a novel treatment to deal with RA, this has been named Boron Neutron Capture Synovectomy (BNCS). The idea is to use a 10B-loaded compound that is mainly absorbed by the synoviocytes or synovial lining cells in the joint, next a beam of neutrons causes the nuclide to fission releasing two high-LET, high-RBE particles that travel distances less than the diameter of a cell. Thus the 10B(n,α)7Li reaction delivers intense radiation damage to the inflamed synovium resulting in synovial ablation. BNCS require no rehabilitation and there is no radiation hazard associated with leakage of the injected compound out of the joint [6].

Studies have been carried out to design a neutron field useful in BNCS using an accelerator [6] and an isotopic neutron source [9, 10]. Yanch et al. have designed a neutron beam and did calculate the dose in a model of knee joint, were the synovial fluid and synovium elemental composition was assumed like water [6].

A Monte Carlo study was carried out to design a cylindrical heterogeneous moderator for a 239PuBe in the aim to produce a thermal neutron field. Analyzed moderators were light water, graphite, lucite, and polyethylene in heterogeneous combination with heavy water. Neutron spectrum produced by the 239PuBe source inside the heterogeneous moderator made with polyethylene and heavy water was use as source term to determine the neutron spectra at several points inside a knee joint model. Neutron spectra were utilized to calculate the dose and dose equivalent.

Also in this investigation neutron kerma factors were calculated in the different components of knee joint. Here the elemental composition of synovial fluid and synovium was assumed similar to blood. During neutron kerma calculation two different concentration of boron was added to synovium.

 

 

 

Materials and Methods


A cylindrical heterogeneous moderator made with heavy water and light water, heavy water and graphite, heavy water and lucite, and heavy water and polyethylene were studied to determine the neutron spectra produced when a 239PuBe neutron source is located inside the moderators. This is shown in Figure 1, where the heavy water volume was 26 liters.

  FIGURE 1.
Heterogeneous moderator

   

Calculations were performed with MCNP 4B code [11], cross sections were taken from ENDF-B/VI library [12]. Neutron spectra were calculated at 27 cm from the center of 239PuBe source, at 0 degrees (along heavy water´s direction) and at 180 degrees (in the direction of complementary moderator). Neutron spectrum produced by the polyethylene and heavy water moderator was utilized as source term to calculate the neutron spectra in a knee joint model, this is shown in Figure 2.

  FIGURE 2.
Knee model.

   

In this model the muscle was modeled as soft tissue and the synovium and synovial fluid were modeled with the elemental composition of blood. The elemental composition utilized in the knee model is shown in Table 1, this data were taken from Seltzer and Berger [13].

TABLE 1.
Elemental composition, in weigth fraction, utilized during modeling.

 -
H
N
O
Na
Si
S
CI
K
Ca
Fe
P
Synovium and synovial fluid
10.1866
2.964
75.9414
0.185
0.003
0.185
0.278
0.163
0.006
0.046
-
Bone tissue
4.7234
4.199
44.6096
-
-
0.315
-
-
20.993
-
-
Muscle
10.1997
3.500
72.9003
0.080
0.500
-
-
0.300
-
-
-
Skin
10.0588
4.642
61.9002
0.007
-
0.159
0.267
0.085
0.015
0.001
0.033
Water
14.372
-
85.628
-
-
-
-
-
-
-
-


In this calculation knee model was irradiated with a disk shape source term. Neutrons are emitted to the knee model. The neutron spectra produced in different locations inside the knee joint model were calculated, this locations, the neutron source and knee model are shown in Figure 3. Calculated neutron spectra were utilized to determine the dose and the equivalent dose inside the knee model using the fluence-to-dose factor and the fluence-to-dose equivalent factor. [14]

Elemental kerma neutron factors from Caswell et al. [14] were utilized to calculate the neutron kerma in the synovium with and without boron. Two concentrations, 0.046 and 0.278 % in weight, of boron were utilized. The results were compared with neutron kerma in the synovium whose elemental composition was alike water.

FIGURE 3.
Source term and knee model. Letters represent the locations where the neutron spectra were calculated

   



 

 

 

Results


The 239PuBe-neutron source inserted in the heterogeneous moderators produce neutron spectra where thermal neutron group is the larger, this spectra are shown in Figure 4.

FIGURE 4.
Neutron spectra from 239PuBe source inserted in the moderators.

   

From this figure polyethylene/heavy water and lucite/heavy water arrays produce the larger amount of thermal neutrons, while graphite/heavy water moderator shows the smaller amount of thermal neutrons and the larger amount of epithermal and fast neutrons.

In Table 2 are shown the thermal neutron fluence and total fluence per neutron emitted by the source, at 27 cm and at 0 and 180 degrees, from the 239PuBe located inside the heterogenous moderators. Neutron fluence along 0 degrees is those neutrons coming out from heavy water section.


TABLE 2.
Neutron spectra features from a 239PuBe source inside moderators.

Moderators
Angle
Thermal neutron fluence
[ n-cm-2 ]
Total neutron fluence
[ n-cm-2 ]
Thermal neutrons
[%]
H2O/D2O
0o
5.73E(-5) ± 1.84%
1.23E(-4) ± 1.31%
46.46
H2O/D2O
180o
0.84E(-5) ± 4.80%
2.55E(-5) ± 4.82%
32.87
Polyethylene/D2O
0o
6.07E(-5) ± 2.13%
1.20E(-4) ± 1.41%
50.37
Polyethylene /D2 O
180o
0.35E(-5) ± 1.48%
1.39E(-5) ± 1.46%
25.18
Graphite/D2O
0o
3.31E(-5) ± 2.95%
1.30E(-4) ± 1.36%
25.42
Graphite/D2O
180o
0.53E(-5) ± 1.29%
1.29E(-4) ± 1.26%
4.09
Lucite/D2O
0o
6.06E(-5) ± 2.09%
1.29E(-4) ± 1.33%
47.06
Lucite/D2O
180o
2.11E(-5) ± 4.61%
3.76E(-4) ± 4.73%

5.60

From this table can be noticed that the larger amount of thermal neutrons emerge form the heavy water section. Polyethylene/heavy water and lucite/heavy water produces the larger amount of thermal neutrons. At 180 degrees neutron leaking out from polyethylene/heavy water moderator are larger than those leaking out from lucite/heavy water moderator. The knowledge of neutron spectrum emerging from other parts of moderators' container, like at 180 degrees, is useful during shielding design. Due to the features shown in table 2, the neutron spectrum produced by the polyethylene/heavy water moderator was utilized as source term to irradiate the knee model.

Neutron spectra at the synovium produced by the 239PuBe source inserted in the polyethylene/D2O moderator are shown in Figure 5. Here the largest spectrum is observed in the point c that is closer to the source term. In all spectra the largest component is the thermal group, this is important because the presence of 10 B inside the synovium will produce the (n,α) reactions. This result can be improved if a neutron reflector is utilized [6, 10]. The dosimetric features per neutron emitted by the source term due to neutrons inside the knee model are shown in Table 3.

  FIGURE 5.
Neutron spectra inside the synovium.

    

TABLE 3.
Dosimetric features of neutron spectra inside the knee joint model.

Point
Average energy
[ MeV ]
Absorbed dose
[ cGy-cm-2 ]
Equivalent dose
[ cSv-cm-2 ]
Quality factor
[ Sv/Gy ]
a
0.116
2.694E(-12)
1.210E(-11)
4.491
b
0.140
6.745E(-12)
2.859E(-11)
4.239
c
0.120
1.860E(-11)
8.018E(-11)
4.311
d
0.145
2.150E(-11)
9.852E(-11)
4.582
e
0.187
1.110E(-12)
5.456E(-12)
4.915
f
0.373
3.689E(-13)
2.219E(-12)
3.033
g
0.531
2.595E(-13)
1.691E(-12)
6.516
h
0.138
2.679E(-12)
1.199E(-11)
4.476
i
0.128
2.566E(-12)
1.137E(-11)
4.431
j
0.134
2.183E(-12)
9.735E(-12)
4.459
k
0.141
2.662E(-12)
1.198E(-11)
4.500
l
0.126
2.557E(-12)
1.133E(-11)
4.388
m
0.136
2.161E(-12)
9.713E(-12)
4.495
n
0.207
3.221E(-12)
1.696E(-11)
5.265
ñ
0.715
3.672E(-13)
2.558E(-12)
6.966

The largest doses are in the points close to the source. The spectra´s characteristics in sites h and k are almost equal due the symmetry in their locations. Comparing neutron spectra and its average energies in sites c and f can be noticed that neutron spectrum is softer close to neutron source, as we move away the neutron source (site f) epithermal neutrons are removed through thermal energies leaving a peak for neutrons from 0.1 to 14 MeV. Collisions between source neutrons with nuclei in the knee tends to reduce neutron energies therefore epithermal neutron group is reduced increasing the group of thermal neutrons and leaving a high energy neutron peak well defined, therefore mean neutron energy is larger in those points away the neutron source.

Kerma factors in water, synovium and synovial fluid are shown in Figure 6. For energies larger than 0.2 MeV neutron kerma factors are approximately equal regardless the elemental composition, while for thermal neutrons the kerma strongly depend of the elemental composition. Thus for synovium with the blood´s elemental composition the neutron kerma is aproximately 40 times larger than water. Adding 0.045 % in weight of boron kerma neutron is 1250 times larger than water, this value is 7351 larger in percent by weight of boron is increased to 0.278 to synovium elemental composition. This is in agreement to calculations performed by Maughan et al. [15] in different tissues. 

FIGURE 6.
Neutron Kerma factors for water and synovium with and without Boron.

    

 

 

 

Conclusions


Monte Carlo calculations were carried out to obtain thermal neutrons from a 239PuBe-neutron source inserted in a cylindrical heterogenous moderator. Polyethylene/D2O moderator produces, per each neutron emitted by the source, 6.07E(-5) cm-2, where 50.4 % are thermal. Using only 26 liters of heavy water this moderator produce better results than the spherical moderator made with light and heavy water that utilizes 61 liters of heavy water. [9, 10]

With a knee joint model, where the elemental composition of synovium and synovial fluid was modeled as blood tissue, the neutron spectra produced by the cylindrical polyethylene/D2O heterogeneous moderator was used as source term and neutron spectra inside the knee were calculated. The dosimetric features shown that doses were larger and the spectra are softer in points located close to the neutron source, as neutrons are transported inside the joint the mean energy is increased because epithermal neutrons are removed through thermal group. This suggests to use a neutron reflector material [10] to increase the neutron field inside the knee for those sites located away the source term.

Neutron kerma factors for synovium without boron and with two different boron concentrations were calculated. These were compared with neutron kerma factor for water. It was found that kerma factor for thermal neutrons for synovium is 40 times larger than neutron kerma factor for water. These differences are 1250 and 7321 times larger than water's kerma factor for synovium with 0.046% and 0.278% by weight in boron respectively. Modeling the elemental composition of tissues as water tends to underestimate the neutron kerma, therefore it is necessary to determine the elemental composition of tissues affected by RA.

Acknowledgments

This work was supported by CONACyT (Mexico) under contract 31288 U

 

 

 

References


1

Nichols, T.L., Miller, L.F. and Kabalka, G.W. (2003) New insights in the micro-dosimetry of Boron neutron capture therapy using Monte-Carlo technique, Trans. Am. Nucl. Soc., 88: 209-210.
back

2

Barth, R.F., Soloway, A.H. and Fairchild, R.G. (1990) Boron neutron capture therapy of Cancer, Cancer Res., 50: 1061-1070.
back

3

Rakovan, L.J., Blue, T.E., Vest, A.L. (1998) Neutron dosimetry in boron neutron capture therapy using aqueous solutions of lithium acetate., Nucl. Instrum. Meth. Phys. Res. A 414(2-3): 357-364.
back

4

Marashi, M.K. (2000) Analysis of absorbed dose distribution in head phantom in boron neutron capture therapy Nucl. Instrum. Meth. Phys. Res. A 440(2): 446-452.
back

5

Pandey, U., Mukherjee, A., Chaudharg, P:R., Pillai, M.R.A. and Venkatesh, M. (2001) Preparation and studies with 90Y-labelled particles for use in radiation synovectomy, Appl. Radiat. Isot., 55(4): 471-475.
back

6

Yanch, J.C., Shortkroff, S., Shefer, R.E., Johnson, S., Binello, E., Gierga, D., Jones, A.G., Young, G., Vivieros, C., Davison, A. and Sledge, C., (1999) Boron neutron capture synovectomy: Treatment of Rheumatoid Arthritis based on the 10B(n,α)7Li nuclear reaction, Med. Phys., 26(3): 364-375 (1999).
back

7

Siegel, M.E., Siegel, H.J. and Luck, Jr., J.V. (1997) Radiosynovectomy´s clinical applications and cost effectiveness: A review, Semin. Nucl. Med. 27(4): 364-371.
back

8

Johnson, L.S., Yanch, J.C., Shortkroff, S., Barnes, C.L., Spitzer, A.I. and Sledge, C.B. (1995) Beta-particle dosimetry in radiation synovectomy, Eur. J. Nucl. Med. 22(9): 977-988.
back

9

Vega-Carrillo, H.R. and Torres-Muhech, C., Sinovectomía por captura de neutrones, Memorias del primer congreso Iberolatinoamericano y del Caribe de Física Médica, (1998): 254-257.
back

10

Vega-Carrillo, H.R. and Torres-Muhech, C. (2002) Low energy neutrons from a 239PuBe isotopic neutron source inserted in moderating media, Rev. Mex. Fís., 48(5): 405-412.
back

11

Briestmeister, J.F. (editor), MCNP-A general Monte Carlo N-particle transport code, Los Alamos National Laboratory, Report LA-12625-M, (1993).
back

12

Hendricks, J.S.,, Frankle, S.C. and Court, J.D., ENDF/B-VI Data for MCNPTM, Los Alamos National Laboratory, Report LA-12891, (1994).
back

13

NCRP, Protection against neutron radiation, National Council on Radiation Protection and Measurements Report No. 38, (1971).
back

14

Caswell, R.S., Coyne, J.J. and Randolph, M.L. (1982) Kerma factors of elements and compounds for neutron energies below 30 MeV, Int. J. Appl. Radiat. Isot. 33, 1227-1262.
back

15

Maughan, R.L., Chuba, P.J. Porter, A.T., Ben-Joseph, E. and Lucas, D.R. (1977) The elemental compositon of tumors: Kerma data for neutrons., Med. Phys. 24(8): 1241-1244.
back

 

 

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