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 Table of Contents    
ORIGINAL ARTICLE
Year : 2022  |  Volume : 47  |  Issue : 4  |  Page : 381-386
 

Photon interaction coefficients for the colorectal cancer tissue


1 Department of Radiography and Radiological Sciences, Faculty of Allied Medical Sciences, University of Calabar, Calabar, Nigeria
2 Department of Physics, Faculty of Physical Sciences, University of Calabar, Calabar, Nigeria
3 Department of Pathology, University of Calabar, Calabar, Nigeria

Date of Submission13-Apr-2022
Date of Decision28-Nov-2022
Date of Acceptance28-Nov-2022
Date of Web Publication10-Jan-2023

Correspondence Address:
Dr. Emmanuel Okon Esien-Umo
Department of Radiography and Radiological Science, Faculty of Allied Medical Sciences, University of Calabar, Calabar
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jmp.jmp_29_22

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   Abstract 

Purpose: The application of radiotherapy to the treatment of cancer requires the knowledge of photon interaction coefficients such as mass attenuation (μm) and mass energy-absorption coefficients (μen/ρ). Although these coefficients have been determined for different tissues, it is lacking for the colorectal cancer (CRC) tissue in the literature. This study determines the μm and μen/ρ for the CRC tissue within the radiotherapy energy range. Materials and Methods: The CRC tissue from autopsy patients was freeze-dried, grounded into a fine powder, and made into pellets of 1 cm thickness. The elements detected in the CRC tissue using Rutherford backscattering spectrometry were used in XCOM to determine the theoretical values of μm and μen/ρ. The CRC tissue was again exposed to X-rays of energies of 6 and 15MV, respectively, to determine its experimental values of μm and μen/ρ. Results: Elements detected included carbon, oxygen and nitrogen making up 96.67%, high atomic number and trace elements making up the remaining 3.33% fraction of the CRC tissue. Conclusion: The theoretical and experimental μm and μen/ρ values showed a good agreement of about 2% difference between them. These values can be used to simulate the CRC tissue with respect to μm and μen/ρ.


Keywords: Colorectal cancer tissue, elemental composition, mass attenuation coefficient, mass energy-absorption coefficient


How to cite this article:
Esien-Umo EO, Obu JA, Chiaghanam NO, Ugbem TI, Egbe NO. Photon interaction coefficients for the colorectal cancer tissue. J Med Phys 2022;47:381-6

How to cite this URL:
Esien-Umo EO, Obu JA, Chiaghanam NO, Ugbem TI, Egbe NO. Photon interaction coefficients for the colorectal cancer tissue. J Med Phys [serial online] 2022 [cited 2023 Feb 1];47:381-6. Available from: https://www.jmp.org.in/text.asp?2022/47/4/381/367420



   Introduction Top


The application of ionizing radiation in the treatment of malignant tumors requires the knowledge of the photon interaction parameters (PIP) of the tissue such as the mass attenuation (μm) and mass energy-absorption (μen/ρ) coefficients.[1],[2] The number of photons absorbed and transmitted and the radiation dose absorbed by the malignant tumor during radiation therapy can be theoretically evaluated using the μm and μen/ρ, respectively.[2] Information and values of μm and μen/ρ are available for some cancer tissues, but are not reported for the colorectal cancer (CRC) tissue in the literature. The values of μm and μen/ρ for any tissue strongly depend on the photon energy, the elements, and their percentage fraction in the tissue undergoing photon interaction.[2],[3],[4] The utility of each of these parameters also depends on the nature of the application and on the energy range.[5] Several studies on μm and μen/ρ for substances, mixtures, and compounds have been cited in the literature.[6],[7],[8],[9],[10],[11],[12],[13]

Radiation therapy has been used to treat CRC. It shrinks tumors in patients due for surgery. It is also useful for palliative treatment and in conjunction with chemotherapy.[14] Radiation therapy damages the deoxyribonucleic acid of the tumor cells leading to either acute or delayed cell. During radiotherapy, photons interact with the cancer tissue transferring energy to the tumor cells, and in the process, the photon beam undergoes attenuation by absorption, scattering, or transmission.[15] Theoretical simulation requires information on the photon interaction data of any tissue. This study determined the μm and μen/ρ for the CRC tissue at the radiotherapy energies of 6–15MV.


   Materials and Methods Top


The study was conducted in two parts; theoretical and experimental determination of attenuation data for CRC tissue.

Theoretical determination of attenuation data

The CRC biopsy was collected for three calendar months. Only autopsy patients with biopsy proof of CRC evaluated in the Histopathology laboratory of University of Calabar Teaching Hospital, Calabar and autopsy patients who had not undergone chemotherapy and radiotherapy treatments were used for this study. The study lasted for twelve calendar months (August, 2020 – July, 2021). The CRC biopsy was examined by a Fellow of the Medical College of Pathology (FMCPATH) and the Chief Consultant Pathologist in University of Calabar Teaching Hospital, Calabar. The CRC tissues were poorly differentiated and did not have the normal structure or pattern. These samples were from male and female patients within the age bracket of 55 – 70 years. The samples were freeze dried at a vacuum pressure of 10−3 Torr and a temperature of between −30 to −40°C in a freeze dryer.[16] After freeze-drying, the samples were grounded into a fine powder in a crusher and allowed to dry overnight. . The CRC tissue powder was further made into pellets of 1cm thickness by the pelletizing machine. The pellets were then used as targets in Rutherford backscattering spectrometry (RBS) to determine the elemental composition of the CRC tissue. RBS has a good mass resolution for light elements, is nondestructive, and detects elements from beryllium to uranium.[17] A Pelletron particle accelerator with model number 5SDH, installed in the Center for Energy Research and Development, Ife – Nigeria, was used to detect the elements and their percentage fractions in the CRC tissue. The pelletized CRC tissue was loaded into the target compartment of the particle accelerator where the tissue was bombarded with helium particle ion (4He+) of 2MeV backscattered at an angle of about 180°. The detector in the target chamber detected the backscatter particles (elements) in the sample based on the pulse produced by the individual element. The pulse appeared as a signal on the energy spectrum of the particle accelerator determining the elemental composition of the CRC tissue.[17] The elemental composition obtained from RBS was used as input data in XCOM, a web-based photon interaction calculation software designed by the National Institute of Science and Technology, USA,[8] to determine the theoretical values of μm for the CRC tissue in the energy range of 6–15 MV. The μm of a chemical compound or mixture was evaluated from the weighted sum of μm of the constituent elements (mixture rule). The μm for the CRC tissue was determined from XCOM from Equation 1:[18]



where μm - mass attenuation coefficient for the compound

- mass attenuation coefficient for the individual elements in the compound

wi - fractional weight of the elements in the compound.

The mass energy-absorption coefficient (μen/ρ) was obtained from the use of an intermediate quantity called the mass energy-transfer coefficient (μtr/ρ), which is the mass attenuation coefficient (μm) in terms of the theoretical f-factors.[19] The μtr/ρ was obtained from Equation 2:[20]



Where f is the weighted average fraction of the photon energy transferred to the kinetic energy of the charged particles in Compton scattering and pair production. The μen/ρ was obtained from the relationship given in Equation 3:[20]



Where g is the energy due to radiative loss.

Experimental determination of attenuation data

The pelletized CRC tissue was exposed to a narrow beam of X-rays at energies of 6 MV and 15 MV, respectively, from a calibrated radiotherapy unit (Elekta Precise linear accelerator unit). The X-ray beam from the linear accelerator was made into a narrow beam by putting a cylindrically shaped lead collimator of 3 mm diameter with a circular aperture on the tube head. The pelletized CRC tissue was mounted on a sample holder positioned between the linear accelerator and the detector such that the source-sample and sample-detector distances were 30 mm each.[21] The pelletized CRC tissue was then exposed to photon energies at 6 MV and 15 MV. The detector used in this study was the Roentgen-Gamma Dosimeter, Type–RGD 27091, with serial number – 7508-PR-208503001. This detector is a battery-operated ionization chamber dose rate meter that measures continuous X-ray and gamma rays. It also measures pulsed X-rays in the dose rate mode and has a wide measuring range. The mean value of three exposures each for the measurement of both the incident (I) and transmitted (I0) X-ray intensities was used in Equations 4 and 5 to the obtain the experimental values of linear attenuation (μ) and mass attenuation (μm) coefficients:



where x is the thickness of the CRC tissue.

The linear attenuation coefficient (μ) for the CRC tissue was given from Equation 4,[22] while the experimental mass attenuation coefficient (μm) for the CRC tissue was determined from Equation 5:[22]



where ρ is the density of the CRC tissue determined from the direct measurement method of volume and mass. The experimental determination of mass energy-absorption coefficient for the CRC tissue was obtained from Equation 3. Errors in the experimental measurement of mass attenuation and mass energy-absorption coefficients are given in Equation 6,[23] respectively.





The percentage difference (% diff) between the theoretical and experimental μm and μen/ρ values is given by Equations 8 and 9,[23] respectively:





Some of the precautions observed in this study included the wearing of hand gloves and nose mask during the handling the CRC tissue, preserving the CRC tissue sample in a sterile container containing formalin before the freeze drying and avoiding mechanical trauma to the pelletized ssample by covering with very thin cellophane paper.

The ethical approval for this study was given by the Ethics Committee of the Department of Radiography and Radiological Sciences, University of Calabar with Reference number UC/ECRA/20/010.


   Results Top


The results of the elemental composition and percentage fraction and the mass attenuation and mass energy-absorption coefficients of the CRC tissue are presented in [Table 1] and [Figure 1], [Figure 2], [Figure 3], [Figure 4].
Table 1: Relationship of μ/ρ and μen/ρ values for the colorectal cancer tissue with photon energy and tissue volume

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Figure 1: Plot of the elements and their % fractions in the CRC tissue from Rutherford backscattering spectrometry. CRC: Colorectal cancer

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Figure 2: Plot of the XCOM attenuation data for the colorectal cancer tissue. INCOH: Incoherent scattering; PP (INF): Pair production in the nuclear field, PP IEF: Pair production in the electron field; TAC: Total mass attenuation

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Figure 3: Plot of the theoretical and experimental μ/ρ values for the CRC tissue. (μ/ρ TH: Theoretical μ/ρ values; μ/ρ EX: Experimental μ/ρ values). CRC: Colorectal cancer

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Figure 4: Plot of the theoretical and experimental μen/P values for the CRC tissue. (μen/P TH – theoretical μen/P values; μen/ρ EX – experimental μen/P values). CRC: Colorectal cancer

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[Figure 1] depicts the elements and their percentage fraction in the CRC tissue obtained from RBS. A total of 13 elements were detected in the pelletized CRC tissue sample. The elements included carbon (74.85%), oxygen (12.79%), and nitrogen (9.03%); other high-atomic-number elements sodium (Na), phosphorus (P), sulfur (S), chlorine (Cl), potassium (K), magnesium (Mg), and calcium (Ca), as well as trace elements (silicon (Si), iron (Fe), and zinc (Zn) were present. Apart from carbon (C), oxygen (O), and nitrogen (N), other elements had a percentage fraction of 3.3% with Zn having the lowest at 0.03%. Hydrogen (H) was not detected in this study.

[Figure 2] depicts the plot of the partial photon interaction processes and the total attenuation coefficient (TAC) from XCOM against photon energy for the CRC tissue. The partial photon processes include the incoherent scattering (INCOH), pair production in the nuclear field (PP INF), and pair production in the electron field (PP IEF) within the energy range of 6–15MV. The values of INCOH were decreasing with increasing photon energy. Pair production processes were increasing with PP INF showing a stronger increment at higher photon energies compared to PP IEF. Incoherent scattering (INCOH) contributed 76.9% to the TAC and was therefore the dominant interaction within this energy range, hence the decreasing values of TAC with increasing photon energy. Pair production in the nuclear field (PP INF) made 21.2% and PP IEF 1.8% contributions to the TAC. The effect of coherent scattering (COH) and photoelectric absorption (PHA) was negligible (0.1%) in this energy range and were therefore not included in the plot.

[Table 1] shows the relationship of μ/ρ and μen/ρ values for the CRC tissue with photon energy and tissue volume. [Figure 3] and [Figure 4] while comparing the theoretical and experimental values of μm and μen/ρ also confirmed the relationship with photon energy for the CRC tissue. The theoretical and experimental values of μm and μen/ρ for CRC were different at different photon energies. The μm and μen/ρ values for both methods also showed a decreasing trend at increasing photon energies with the experimental values being a little higher at 6 and 15 MV. In both plots, the experimental values of μ/ρ and μen/ρ were closer to the theoretical values at 15MV. The error in the experimental values of μm and μen/ρ was about 0.13%, and the percentage difference between the theoretical and experimental values of μm and μen/ρ for the CRC tissue was about 2%.


   Discussion Top


To our knowledge, this study represents the first attempt at investigating the PIP for the CRC tissue. The present study used elemental composition of the CRC tissue to determine its mass attenuation and mass energy-absorption coefficients. Most works on the elemental composition of the CRC tissue have concentrated on trace elements and heavy metals in the CRC tissue.,[24],[25],[26],[27],[28],[29],[30] In those studies, the elevated levels of Mg, Si Zn, Fe, Ca, K, P, and S were detected in CRC tissues.[24],[25],[26],[27],[28],[29],[30],[31] The percentage fraction of these elements in our study were, however, higher and may be implicated in the development of malignancy in the colorectal tissue. Variations in the elemental composition between this study and the previous ones could be due to the difference in tissue samples, experimental techniques,[2] and age, gender, and the state of health of the person.[2],[32] However, the presence of some elements at certain percentage fractions has been reported to strongly affect the photon interaction processes in tissues.[33] For example, PHA and Compton scattering processes are strongly affected by the percentage fraction of high-atomic-number elements (Z > 8) and hydrogen content, respectively, in tissues.[33]

The decrease in the μm values of the CRC tissue at higher photon energies in the theoretical and experimental methods may have been due to the decreasing trend of the μm values of the dominant element (C-74.85%);[34] the reduction in absorption with increasing incident photon energy which results in increased transmission of photons through the CRC tissue[35],[36],[37] and the dominating decreasing trend of Compton (INCOH) scattering processes within 6 – 15MV energy range [Figure 2]. The μm values obtained from theoretical and experimental methods were different at different photon energies because the partial interaction processes such as the Compton scattering (INCOH) and pair production (in the nuclear-PP INF and in electric field PP-IEF) were also different at different photon energies [Figure 2].

The mass energy-absorption coefficient (μen/ρ) accounts for the loss of energy in the material from photon interactions processes and is therefore very essential in the determination of absorbed doses in biological materials and shielding.[38] The decrease in μen/ρ values at higher photon energies in this study maybe as a result of the dependence of μen/ρ on μm and on the reduction in dose deposition in the CRC tissue at higher photon energies[38] The closer experimental to theoretical values of μm and μen/ρ at 15MV could be attributed to the increasing trend of PP INF, which reduced the effect of INCOH scattering [Figure 2]. The percentage difference of 2% showed that the theoretical and experimental values of μm and μen/ρ were very close indicating a good agreement. This suggests that the μm and μen/ρ values can be used to simulate the CRC tissue within 6–15MV. The error in the experimental values of μm and μen/ρ at 0.13% and the discrepancies in the μm and μen/ρ values between the two methods at 6 and 15MV may be due to the method of sample preparation[9] and on the experimental setup.[37]





This study determined the elemental composition, μm and μen/ρ, for the CRC tissue. The elements detected included C, O and N making up 96.67%, high-atomic-number elements (Na, P, S, Cl, K, Mg and Ca) and trace elements (Si, Mg, Fe and Zn) making up the remaining 3.33% fraction of the CRC tissue. The theoretical μm and μen/ρ values ranged from 0.03493 to 0.01750 cm2/g and 0.02440–0.01680 cm2/g, respectively, within 6–15MV. The experimental values were 0.0254 cm2/g and 0.0249 cm2/g at 6MV, and 0.0178 cm2/g and 1.68 cm2/g at 15 MV for μm and μen/ρ, respectively. A good agreement of about 2% was found between the theoretical and experimental values. These results can be used to simulate the CRC tissue within the radiotherapy range of 6–15 MV with respect to μm and μen/ρ.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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