Estamos en proceso de migración de contenidos al nuevo sitio de Alasbimn Journal. Puede acceder a los últimos artículos en: www.alasbimnjournal.net
Inicio arrow Conclusion. Utilidad del gatillado

Year 12, Nº 47, January 2010 / Año 12, Nº 47, Enero 2010

Neutron KERMA factors of Human Tissues.  Article N° AJ37-5

 

H.R. Vega-Carrillo,1, E. Manzanares-Acuña 1,R. Barquero2,J.L. Gutierrez-Villanueva3, A. Martin-Martin3

1Unidad Académica de Estudios Nucleares e Ingeniería Eléctrica
Universidad Autónoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. México
2Hospital Universitario Río Hortega de Valladolid, Valladolid, Spain
3Laboratorio LIBRA, Edificio I+D, Paseo Belén 3. 47011 Valladolid, Spain.

Correspondencia:

H.R. Vega-Carrillo,       
Unidad Académica de Estudios Nucleares e Ingeniería Eléctrica.
Apdo. Postal 336, 98000 Zacatecas, Zac. México
Fone:     +52-492-922-7043 Ext. 120
Fax:       +52-492-922-7043 Ext. 118
e-mail :fermineutron@yahoo.com

Cita/Reference:
H.R. Vega-Carrillo et al. Neutron KERMA factors of Human Tissues. Alasbimn Journal 9(37): July 2007. Article N° AJ37-5. http://www2.alasbimnjournal.cl/journal/index.php?option=com_content&task=view&id=60&Itemid=1

 

Abstract

A program to calculate the neutron KERMA in human tissues has been developed. The program was developed in Mathcad and contains the neutron kerma factors of those elements that are present in different human tissues. Having the elemental composition of any human tissue the neutron kerma can be easily calculated. The program was tested using the elemental composition of tumor tissues such as sarcoma, melanoma, carcinoma and adenoid cystic. Neutron kerma for adipose and muscle tissue for normal adult was calculated. The results are in agreement with those published in literature. The neutron kerma for water was also calculated because in some dosimetric calculations water is used to describe normal and tumor tissues. From this comparison was found that at larger energies kerma factors are approximately the same, but energies less than 100 eV the differences are large.

Keywords: Neutron KERMA, Neutron background, Human tissue

 

1. Introduction

The Earth's crust contains small amounts of naturally radioactive materials. During disintegration natural occurring elements, like uranium and thorium, produce 226Ra, 222Rn, 210Pb, etc that are transported from the soil to the air and to the water (Godoy and Godoy, 2006).

Gamma and alpha radiation emitted by natural occurring radioactive nuclei in soil and rocks pose a health risk. Galactic cosmic rays are high-energy particles that come from outside the Solar system. Together with the solar cosmic rays, constitutes the radiation environment at high altitudes in the Earth's atmosphere. These primary cosmic rays collide with molecules, atoms and nuclei in the atmosphere producing showers of secondary radiation that reach the surface of the planet (Vega-Carrillo and Manzanares-Acuña, 2004). Thus, gamma-rays, alpha particles and neutrons are components of natural radioactivity.

Several studies have been carried out, in air, soil and water, to study the natural radioactivity of nuclei that emits gamma and alpha particles, but few studies has been published dealing with the neutron natural background. In radiation protection neutron dosimetry is an important issue because the use and application of neutrons in different areas.

Neutrons are present having two natural sources: Cosmic rays and radioactive heavy nuclei ; also are obtained from nuclear reactors, accelerators or isotopic neutron sources. Neutrons are widely utilized in different areas such as, solid state physics, material science, chemical analysis, nuclear physics and neutron therapy (Tsuda et al. 2003).

Although there are on going several research projects to treat cancer, radiotherapy with photon and electron beams is still the most diffused procedure to control and eliminate tumor diseases (Lenox, 2001, Ognaro et al. 2000). The patient is therefore exposed to an undesirable radiation composed of secondary Compton, pair production and bremstrahlung secondary photons, as well as to direct and reflected neutrons, which produce a non-negligible dose (Lin et al. 2001). These neutrons are produced through (γ, n) reactions between the hard x-rays and the nuclei in the accelerator head, treatment room and the patient body. (Barquero et al. 2005)

To estimate the radiation dose in human body exposed to external sources a set of simple specifications of mass, dimensions, and elemental composition of the organs and tissues are required (ICRU, 1994).

Measurement of the absorbed doses within and around irradiated body tissues necessitates selected materials from which phantoms and radiation detectors are constructed. These materials should be, in density and elemental composition, as close as possible to organs and tissues of human body (ICRU, 1989).

Neutron-induced reactions play an important role in the particle transport, radiation effects in accelerator-based systems for transmutation, medicine and material research. Anthropogenic and natural occurring charged-particle neutron production reactions are important to estimate the absorbed dose in living tissues where the neutron energy is transferred to kinetic energy of charged particles per unit mass (Kerma). This information is also required for the evaluation of the radiation effects and the nuclear heating (Hirasawa et al. 1999).

To determine the dose deposition is important to know the element concentration of body tissue and tissue substitute. In the case of radiation therapy treatment with photons, electrons, and neutrons this relevance has been discussed in reports published by the International Commission on Radiation Units and Measurements (ICRU, 1989, ICRU, 1992).

Kerma per unit fluence, named Kerma coefficients o sometimes called fluence-to-kerma factors, are useful in neutron dosimetry because absorbed dose measurements are realized with instruments build with tissue-equivalent materials but hardly ever have the exact composition of the tissue in which the kerma or absorbed dose is measured. Thus, if there is some approximate knowledge of the neutron spectrum at the point of measurement, the kerma or absorbed dose in the tissue can be found from the measured kerma or absorbed dose in the instrument. Also, if the neutron spectrum is known at a point of interest, from either measurements or calculations, the kerma is the product of the fluence and the appropriately averaged kerma factor (ICRU, 1977).

Particularly ICRU 46 (1992) includes composition data of a set of selected organs for several different individuals. However, it does not include relevant data of tumors of different histologies (Maughan et al. 1997).

In this study neutron kerma coefficient for a set of elements important in biology and medicine has been utilized in a program in the aim to calculate the neutron kerma factors for different tissues.

 

2.Materials and Methods

A series of kerma coefficients for elements (kf(E)) were used to calculate the kerma factors for different tissues (kT(E)) using the equation 1.

                                                                              kT(E) = Σi wikf (E)i             (1)

here, wi is the percent composition by weight of i-th element in tissue and kf(E)i is the kerma coefficient of i-th element in the tissue. Calculated factors were compared with those reported in literature for water, normal body tissues and selected tumors.

Kerma coefficients in function of neutron energy for elements that are relevant in biology were taken from Caswell et al. (1982). In figure 1 are shown the kerma coefficients for C, H, O and N, while in figure 2 kerma coefficients for Na, Mg, K, and Cl. Kerma coefficients for B, Al, Si, and P are plotted in figure 3 and in figure 4 the kerma coefficients for S, Ca, and Fe are shown.

Figure 1: Neutron KERMA coefficients for C, H, O and N
Figure 1: Neutron KERMA coefficients for C, H, O and N
 
Figure 2: Neutron KERMA coefficients for Cl, K, Na and Mg
Figure 2: Neutron KERMA coefficients for Cl, K, Na and Mg
 
Figure 3: Neutron KERMA coefficients for B, Al, P and Si.
Figure 3: Neutron KERMA coefficients for B, Al, P and Si.
 
Figure 4: Neutron KERMA factors for S, Ca and Fe
Figure 4: Neutron KERMA factors for S, Ca and Fe

To test the program the elemental composition of sarcoma, melanoma, carcinoma, adenoid cystic, adipose and muscle tissues were calculated. To compare the neutron kerma of tumor and normal tissues the kerma of water was calculated as well. In table 1 the weight fraction of elemental composition of these tissues are shown. These tumor types are histologies, which are known to respond well to neutron therapy.

Table 1: Elemental composition, in weight fraction, of selected tumor and normal body tissues.

Tissue

H
[w/o]

C
[w/o]

N
[w/o]

O
[w/o]

Other
[w/o]

Carcinoma

10.0

18.5

4.2

65.9

Ash

Sarcoma

10.5

8.1

2.1

78.0

Ash

Melanoma

9.4

21.2

5.6

61.5

Ash

Adipose

11.2

51.7

1.3

35.5

0.1 Na, 0.2 P, 0.3 S, 0.2 Cl, 0.2 K

Muscle

10.2

14.3

3.4

71.0

0.1 Na, 0.2 P, 0.3 S, 0.1 Cl, 0.4 K

 

3. Results and Discussion

To calculate kerma factor using equation 1 a program, named Kerma, was developed in Mathcad (Mathsoft, 1993). This program uses the kerma coefficients of elements to calculate the neutron kerma factors of any material or tissue providing the element composition by weight fraction. In figure 5 and 6 are shown sections of Kerma program. The output of calculated neutron kerma factors are given out by the program in plots, tables and text files. The program is available through the corresponding author.

Figure 5: Presentation and loading data in Kerma program
Figure 5: Presentation and loading data in Kerma program
 
Figure 6: Loading data in Kerma program
Figure 6: Loading data in Kerma program

In figure 7 the neutron kerma factors for tumor tissues of sarcoma and melanoma are shown, in this figure the neutron kerma for normal adult adipose tissue and water are added for comparison.

Figure 7: Neutron KERMA factors of water, sarcoma, melanoma and adipose tissues.
Figure 7: Neutron KERMA factors of water, sarcoma, melanoma and adipose tissues.

In figure 8 the neutron kerma factors for tumor tissues of carcinoma and adenoid cystic are shown. Here, the neutron kerma for normal adult muscle tissue and water are added for comparison.

Figure 8: Neutron KERMA factors of water, carcinoma, adenoid cystic and muscle tissues.
Figure 8: Neutron KERMA factors of water, carcinoma, adenoid cystic and muscle tissues.

In both figures can be noticed that for neutrons with energies less than 10-4 MeV there are significant differences in kerma factors, this differences are larger when normal and tumor tissues are compared with water. This is particularly important because to simplify dosimetric calculations tumor and normal tissues are sometimes described as water-made.

 

4. Conclusions

 

Calculated neutron kerma factors for normal and tumor tissues are in agreement with those reported in literature (Maughan, 1997).

For fast neutron therapy the energies of interest are between 1 and 70 MeV where differences between neutron kerma of tumor and normal tissues are small, however these differences are large for neutrons with energies less than 100 eV.

During neutron dosimetry is important to know the elemental composition of different tissues exposed to neutrons. This information allows estimate the kerma factors in function of neutron energy. If a neutron spectrum is available neutron kerma can be calculated using the neutron kerma factor of different tissues.

The Kerma program here developed is user friendly and simple to execute. The program can be easily updated if new kerma factors are available or in case if instead of human tissues the neutron kerma of materials wants to be estimated.

Acknowledgments

This work is part of SYNAPSIS research project partially supported by CONACyT (Mexico) under contract SEP-2004-C01-46893.

 

References

Baquero, R., Edwards, T.M., Iñiguez, M.P., Vega-Carrillo, H.R., 2005. Monte Carlo simulation estimates of neutron doses to critical organs of a patient undergoing 18-MV x-ray LINAC-based radiotherapy. Medical Physics 32: 3579-3588. back

Caswell, R.S., Coyne, J.J. and Randolph, M.L., 1982. Kerma factors of elements and compounds for neutron energies below 30 MeV. International Journal of Applied Radiation and Isotopes 33, 1227-1262back

Godoy J.M. and Godoy, M.L., 2006. Natural radioactivity in Brazilian groundwater. Journal of Environmental. Radioactivity 85, 71-83. back

Hirasawa, Y., Baba, M., Nauchi, Y., Ibaraki, M., Miura, T., Hiroishi, T., Aoki, T., Nakashima, H., Meigo, S-I and Tanaka, S., 1999. Measurements of double differential charged-particle production cross-sections for 55, 65, 75 MeV neutrons, Proceedings of the 1999 Symposium on Nuclear Data, JAERI Conference 2000-005. Tokai, Japan, 18-19th November, paper 3.20. back

ICRU, 1977. Neutron dosimetry for Biology and Medicine, International Commission on Radiation Units and Measurements Report 26, Pergamon Press, Washington, DC, USA. back

ICRU, 1989. Tissue substitutes in radiation dosimetry and measurements, International Commission on Radiation Units and Measurements Report 44, Pergamon Press, Washington, DC, USA. back

ICRU, 1992. Photon, electron, proton and neutron interaction data for body tissues, International Commission on Radiation Units and Measurements Report 46, Pergamon Press, Washington, DC, USA. back

ICRU, 1994. Report of the task group on reference man, International Commission on Radiological Protection Report 23, Pergamon Press, Oxford, UK.   back

Lenox, A.J., 2001. Accelerators for cancer therapy, Radiation Physics and Chemistry 61, 223-226. back

Lin, J-P., Chu, T-Ch., Lin, S-Y., and Liu, M-T., 2001. The measurement of photoneutrons in the vicinity of a Siemens Primus linear accelerator. Applied Radiation and Isotopes 55, 315-321. back

MathSoft, 1993. Mathcad4.0 user's guide, MathSoft Inc. Cambridge Massachusetts USA.  back

Maughan, R.L., Chuba, P.J., Porter, A.T. and Ben-Joseph, E., 1997. The elemental composition of tumors: Kerma data for neutrons, Medical Physics 24, 1241-1244.  back

Ognaro, C., Zanini, A., Nastasi, U., Ródenas, J., Ottaviano, G. and Manfredoni, C., 2000. Analysis of photoneutrons spectra produced in medical accelerators, Physics Medicine and Biology 45, L55-L61.  back

Tsuda, S., Endo, A. and Yamaguchi, Y., 2003. Synthesis and characterization of soft-tissue substitute for neutron dosimetry, Journal of Nuclear Science and Technology 40, 1027-1031.  back

Vega-Carrillo, H.R. and Manzanares-Acuña, E., 2004. Background neutron spectrum at 2420 m above sea level. Nuclear Instruments and Methods in Physics Research A 524, 146-151.  back

Vega-Carrillo, H.R. and Manzanares-Acuña, E., 2003. Neutron source design for Boron Neutron Capture Synovectomy. Alasbimn Journal 6(22): October 2003. Article No AJ22-8 524, 146-151. <back

 
< Anterior / Prev   Siguiente / Next>
Creative Commons License
Esta obra est� bajo una licencia de Creative Commons
© 2020 Alasbimn Journal
Joomla! es Software Libre distribuido bajo licencia GNU/GPL.
Admin

Alasbimn Journal ISSN.  0717-4055