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ORIGINAL ARTICLE
Year : 2015  |  Volume : 38  |  Issue : 3  |  Page : 83-91  

Comparison of effective atomic numbers of the cancerous and normal kidney tissue


Department of Physics, Government College for Women, Kolar, Karnataka, India

Date of Web Publication10-Nov-2015

Correspondence Address:
H C Manjunatha
Department of Physics, Government College for Women, Kolar - 563 101, Bengaluru, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.169376

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  Abstract 

The effective atomic number (Z eff ) and electron density (N e ) of normal kidney and cancerous kidney have been computed for total and partial photon interactions by computing the molecular, atomic, and electronic cross section in the wide energy range of 1 keV-100 GeV using WinXCOM. The mean Z eff and N e of normal kidney and cancerous kidney in the various energy ranges and for total and partial photon interactions are tabulated. The variation of effective N e with energy is shown graphically for all photon interactions. In addition to this computer tomography (CT), numbers of normal kidney and cancerous kidney for photon interaction and energy absorption is also computed. The role of Z eff in the dual-energy dividing radiography is also discussed. The values of Z eff and N e for cancerous kidney are higher than normal kidney. This is due to the levels of elements K, Ca, Fe, Ni, and Se are lower and those of the elements Ti, Co, Zn, As, and Cd are higher in the cancer tissue of kidney than those observed in the normal tissue. The soft tissue and cancerous tissue are very similar, but their atomic number differs. The cancerous tissue exhibits a higher Z eff than the normal tissue. This fact helps in the dual-energy dividing radiography which enables to improve the diagnosis of the kidney cancer. Hence, the computed values may be useful in the diagnosis of the kidney cancer. CT numbers for normal kidney are higher than cancerous kidney.

Keywords: Cancerous kidney, effective atomic number, electron density, kidney


How to cite this article:
Manjunatha H C. Comparison of effective atomic numbers of the cancerous and normal kidney tissue. Radiat Prot Environ 2015;38:83-91

How to cite this URL:
Manjunatha H C. Comparison of effective atomic numbers of the cancerous and normal kidney tissue. Radiat Prot Environ [serial online] 2015 [cited 2019 Jun 20];38:83-91. Available from: http://www.rpe.org.in/text.asp?2015/38/3/83/169376


  Introduction Top


The photon has started to be used in different fields of medicine such as therapy, diagnosis, and dosimetry. In order to keep radiation hazards within the desired limit, the radiation absorption mechanism in materials should be known. This can be represented by some quantity of materials such as mass attenuation coefficients (μ/ρ), effective atomic number (Z eff ), and effective electron density (N e ). The attenuation coefficient is defined as the probability of a radiation interacting with a material per unit path length. The linear attenuation coefficient for a material depends on the incident photon energy, the atomic number, and the density of the material. A single number cannot represent the atomic number in composite materials. Thus, a number called the "Z eff" would be defined for such materials. The measurement or calculation of Z eff is a pioneering step for many fields of scientific applications, and it provides conclusive information about the material with the radiation interacts. Grinyov et al. [1] developed dual-energy radiography for separate detection of materials differing in their Z eff and local density. This number is also very useful to visualize a number of characteristics of a material for technological, nuclear industry, space research programs, and engineering in many fields of scientific and biological applications. The other important quantity is the effective electron number or N e , and it is defined as the electrons per unit mass of the absorber. [2]

Although the cancerous tissue and the soft tissue are very similar, their atomic number differs. In the dual-energy image, weighted subtraction of the logarithm of the low energy image from that of the high-energy image is proportional to the product of the atomic number, density, and thickness of the tissue. [3],[4],[5],[6] Cancer is a major health problem. There have been some attempts to understand the role played by trace elements in either initiating or promoting or inhibiting the growth of cancer. [7],[8],[9],[10] In such investigations, the concentrations of different elements in the tissues of the cancer-afflicted organ, as well as the normal tissue of the same organ are measured employing a high-precision technique like proton-induced X-ray emission (PIXE). Johansson et al.[11] was used 1 st time PIXE technique for the multielemental analysis. The elemental concentrations in the normal and cancer tissues of the kidney, as well as the stomach, are measured employing PIXE technique by Reddy SB, et al. [11]

So far, to our knowledge, no study has been done for the comparison of Z eff of normal tissue with cancerous tissue. This prompted us to undertake a rigorous and exhaustive investigation of photon interaction parameters like Z eff , N e , and computer tomography (CT) number in normal kidney and cancerous kidney for wide energy region 1 keV-100 GeV using WinXCOM program.[12],[13] In this work, an attempt has been made to compare the Z eff , N e and CT number of normal kidney and cancerous kidney. Such data will be prime importance in radiography and dose calculations, which is useful in radiation medicine.

Present work

Computation of effective atomic number


When a beam of photons passes through an absorber, the photons interact with the atoms and are either absorbed (photoelectric effect, pair and triplet production, and photonuclear), or scattered away from the beam (coherent and incoherent scattering). The intensity of the transmitted beam of photons is the sum of the cross sections per atom for all the above processes. In this work, the μ/ρ and photon interaction cross sections in the energy range from 1 keV-100 GeV are generated using WinXCom. [12],[13] This program uses the same underlying cross-sectional database as the well-known tabulation of Hubbell and Seltzer. [14] The total molecular cross section (σm) is computed from the following equation using the values of μ/ρ ([μ/ρ]bio). [15],[16],[17],[18]



where n i is the number of atoms of i th element in a given molecule, (μ/ρ)bio, the μ/ρ of biomolecule, N, the Avogadro's number and A i , the atomic weight of element i. (μ/ρ)bio was estimated based on the chemical composition [11] given in [Table 1]. The atomic cross section (σa ) is estimated using the equation:
Table 1: Elemental composition (in percentage) of normal and cancerous kidney


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The effective electronic cross section (σe ) is computed from μ/ρ (μ/ρ) i of i th element in the given molecule



Where, f i is the fractional abundance (a mass fraction of the i th element in the molecule), and Z i is the atomic number of the i th element in a molecule. Finally, the Z eff is estimated as



Calculation of electron density

The effective N e, N e, expressed in the number of electrons per unit mass is closely related to the Z eff . For a chemical element, the N e is given by N e = NZ/A. This expression can be generalized to a compound,



In this work, N el for different photon interactions is estimated using Equation (5).

Calculation of computer tomography number

The CT number is a normalized value of the calculated X-ray absorption coefficient of a pixel (picture element) in a computed tomogram, expressed in Hounsfield units. The CT number for total photon interaction is calculated using the equation given by Thomos.[19]



μm and μw are energy attenuation coefficient of given material and water, respectively.


  Results and discussions Top


The variation of N el for different photon interactions of normal kidney and cancerous kidney is as shown in [Figure 1] [Figure 2] [Figure 3] [Figure 4] [Figure 5] [Figure 6]. The variation of N e with photon energy for total photon interactions is as shown in [Figure 1] and this variation is because of the dominance of different photon interactions. There is a slight increase in the N el up to 4 keV and becomes maximum then decreases sharply. It remains constant from 0.2 MeV to 2 MeV, which shows that coherent and incoherent processes increases. From 2 MeV to 300 MeV, there is a regular increase in the N e with photon energy. This is due to the increase in incoherent and pair production processes. From 300 MeV onward N e remains constant, which is due to dominance in pair production processes.
Figure 1: Variation of effective electron density with photon energy for the total photon interaction (with coherent). (1) Normal kidney (2) cancerous kidney

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Figure 2: Variation of effective electron density with photon energy for photoelectric absorption. (1) Normal kidney (2) cancerous kidney

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Figure 3: Variation of effective electron density with photon energy for incoherent scattering interaction. (1) Normal kidney (2) cancerous kidney

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Figure 4: Variation of effective electron density (Ne) with photon energy for pair production in nuclear field. (1) Normal Kidney (2) Cancerous Kidney

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Figure 5: Variation of effective electron density (Ne) with photon energy for pair production in electronic field. (1) Normal Kidney (2) Cancerous Kidney

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Figure 6: Variation of effective electron density (Ne) with photon energy for sum of non coherent process. (1) Normal Kidney (2) Cancerous Kidney

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The variation of N e with photon energy for photoelectric absorption is as shown in [Figure 2] and this shows that N e increases gradually up to the photon energy 1 MeV. It remains constant thereafter, i.e., independent of photon energy. This is due to the dominance in photoelectric absorption in low energy region, i.e., <1 MeV and for the substances of higher atomic number (Z) than for low Z substances. [Figure 2] shows a small peak around 1 MeV for cancerous kidney because of the presence of trace elements of high Z values (whose Z values are up to 82). This curve is a signature of the presence of high Z-trace elements in the cancerous kidney. This peak is not there in the curve for a normal kidney. The variation of N e with photon energy for incoherent scattering is as shown in [Figure 3] and it indicates that N e increases from 1 keV to 1 MeV shows that it depend on energy. This variation is because of the proportion and the range of atomic numbers of the elements present in the normal kidney and cancerous kidney. Above 1 MeV, N e remains constant and independent of energy for both normal kidney and cancerous kidney. The variation of N e with photon energy for pair production in the nuclear field is as shown in [Figure 4], and it shows that Ne slightly decreases trend with an increase in photon energy. It may be due to the fact that pair production in the nuclear field is Z 2 dependent. The variation of N e with photon energy for pair production in the electric field is as shown in [Figure 5]. It shows that Ne is independent of photon energy from 3 MeV to 30 MeV. From 30 MeV, Ne decreases with the increase of photon energy up to 15 GeV and then it is independent of energy. [Figure 6] shows the variation of Ne with photon energy for the sum of noncoherent process, which shows almost similar trend as in the case of total photon interaction.

The variation of Z eff with a photon energy of both normal kidney and cancerous kidney for total and partial photon interaction processes are similar to that of N e , which can be explained in the similar manner. The mean Z eff and N e of normal kidney and cancerous kidney for various energy ranges and for different interaction process are as shown in [Table 2] and [Table 3]. The values of Z eff and N e for cancerous kidney are higher than normal kidney. This is due to the levels of elements K, Ca, Fe, Ni, and Se are lower and those of the elements Ti, Co, Zn, As, and Cd are higher in the cancer tissue of kidney than those observed in the normal tissue. The estimated CT numbers for normal kidney and cancerous kidney at various photon energies from 1 keV to 100 GeV are tabulated in [Table 4] and [Table 5]. CT numbers for cancerous kidney are higher than normal kidney. This shows that CT image for cancerous affected kidney sections is lighter than the normal kidney sections.
Table 2: Mean Zeff of normal kidney and cancerous kidney for various energy ranges and for different interaction process


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Table 3: Mean effective electron density Ne (×10+24 electrons/g) of normal kidney and cancerous kidney for various energy ranges and for different interaction process


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Table 4: CT - numbers in normal kidney


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Table 5: CT - numbers in cancerous kidney


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Traditional X-ray radiography visualizes the distribution of the photon number which is registered by a detector or film pixel. The number of counted photons is determined as:



Where N 0 is initial photon number generated by X-ray tube which comes through the elementary part of the tissue, μ is the μ/ρ (the sum of mass scattering and mass absorption coefficients), which is determined by an Z eff and X-ray energy, ρ is the density of the elementary part of the tissue, d is the thickness of the tissue, "S" is the part of photons scattered by the neighboring parts of the target tissue and by the radiography equipment, "a" is the portion of photons absorbed by the radiography equipment, g is the portion of photons that reached and interacted with the detector or the film. Let us denote the division and subtraction of the mass absorption coefficients as:



Where, μL and μH are the mass coefficients of absorption for low- and high-energy, respectively. The subtraction of the logarithms is proportional to the product of the Z eff , density, and thickness.



This dual-energy subtraction radiography may provide information regarding the product of the atomic number, density, and thickness of the target tissue. Dual-energy subtraction radiography is based on weighted subtraction of the logarithm of the low energy image from that of the high-energy image. But division of the logarithms is proportional only to the Z eff :



This dual-energy dividing radiography is based on dividing of the logarithms of the low energy image by the high-energy image. That ratio depends upon only a Z eff and does not depend on the density and thickness of the tissue. The division of the same logarithms can be more useful in diagnosing malignant growths because it depends only on the Z eff . [20],[21] The product of average density and thickness can be calculated as:



As a rule, on the most part, the thickness of the target tissue is constant during radiography. Hence, that expression allows us to extract the real density distribution. Subtraction and division of the logarithms give three additional images: The Z eff , the density, and the product of the two. In the dual-energy image, coordinates of darker images correspond to healthy sections of the kidney and coordinates of lighter images correspond to cancerous affected kidney tissue. This is due to the fact that the attenuation coefficient of normal kidney tissue is higher than the cancerous affected kidney tissue. This is also due to the fact that the values of Z eff and N e for cancerous kidney are higher than normal kidney. Hence, the distribution of the Z eff provides the opportunity to investigate the dynamic of its evolution and effectiveness of medical treatment. Invariant visualization of the Z eff on the base of dual-energy dividing radiography enables improving the diagnosis of the kidney cancer. Hence, the computed values may be useful in the dual-energy dividing radiography.

Previous worker [22] calculated linear attenuation coefficient for kidney using Monte Carlo program and other theoretical method. King et al. [23] measured the linear attenuation coefficient over the range 30-110 keV for Kidney. The μ/ρ calculated by the present method are converted to linear attenuation coefficients using the density of 1.05 g/cm 3 from International Commission on Radiological Protection [24] for kidney. The calculated linear attenuation coefficients for kidney using present method is compared with the previous work, and this comparison is as shown in [Table 6]. This comparison shows that the values calculated in the present method agree with the measured and other theoretical data in literature.
Table 6: Comparison between calculated and measured values of linear attenuation coefficients (cm−1) for kidney tissue


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  Conclusions Top


From this study, it can be concluded that:

  • The values of Z eff and N el for cancerous kidney are higher than normal kidney. This is due to the levels of elements K, Ca, Fe, Ni, and Se are lower and those of the elements Ti, Co, Zn, As, and Cd are higher in the cancer tissue of kidney than those observed in the normal tissue
  • Even though the soft tissue and cancerous tissue a very similar, but their atomic number differs. The cancers exhibit a higher Z eff than the normal tissue. This fact helps in the dual-energy dividing radiography, which enables to improve the diagnosis of the kidney cancer. Hence, the computed values may be useful in the diagnosis of the kidney cancer
  • The subtraction of the logarithm of a number of counted photons in radiography is proportional to the Z eff . In other words, regions of brightness in radiography are proportional to the Z eff . The values of Z eff for cancerous kidney are higher than normal kidney for the energy range 0.001-0.1 MeV. Thus, cancer affected kidney sections in the image are brighter than that of normal kidney sections.
Finally, the cancers exhibit a higher Z eff than the normal tissue. This fact helps in the dual-energy dividing radiography which enables to improve the diagnosis of the kidney cancer. Hence, the computed values may be useful in the dual-energy dividing radiography.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Grinyov B, Ryzhikov V, Lecoq P, Naydenov S, Opolonin A, Lisetskaya E, et al. Dual-energy radiography of bone tissues using ZnSe-based scintielectronic detectors. Nucl Instrum Methods A 2007;580:50-3.  Back to cited text no. 1
    
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[PUBMED]  Medknow Journal  
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]


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