Introduction

Radiation therapy is a treatment that plays a key role in the fight against cancer [1]. This radiation can be applied from outside the patient's body (remote treatment) or by placing radioactive materials in the patient's body (brachytherapy). In remote therapy, linear medical accelerators are used to obtain photons or electrons energy in the MeV range, as well as proton beams and heavy ions. Photons in this energy range allow the treatment of tumors in the body. The radiation fields are formed by a collimator system on the linear accelerator (linac) head, creating fields that are formed separately for the target treatment volume. The accelerator head is mounted in such a way that it can rotate around the patient at any desired angle. In this way, the rays can be irradiated from different directions, thus maintaining healthy tissue and important organs and focusing on the tumor tissue [2]. Previous research has shown that Monte Carlo methods are suitable for simulating photon beam transport in linear medical accelerators. Simulation methods can be used to measure the dose distribution in phantoms and patients' body. Geant4 toolkit [3] and MCNPX code were used to determine the phase space specification of the 6 MeV Elekta compact accelerator. MCNPX code based on the Monte Carlo method is capable of transporting 32 atomic particles and nuclei in different environments [4]. Geometry and Tracking toolkit (Geant4) is a simulation software based on the Monte Carlo method that written in C++ software and tracks all kinds of particles and has the ability to simulate magnetic and gravity fields and photon transport in nanoscale. Geant4 has many libraries with different particle cross sections in different environments [5]. In this method, the spatial, angular, and energy spectra of X-ray colliding with the tungsten target are extracted and its information is introduced as phase space. The spectral information of the phase space is defined as the X-ray source in the Geant4 toolkit and MCNPX code of the Elekta Compact accelerator for calculation of total scattering coefficient (Scp). Scp is not calculated for the compact model Elekta accelerator till now.

Scattering factor calculations show the scattering of photons from the accelerator components and the patient's body. Therefore, it is possible for the out-of-field dose to reach other parts of the body in the treatment of cancer in a particular organ. To examine out-of-field dose, a Medical Internal Radiation Dose (MIRD) phantom [6] is defined on the patient's bed in treatment room under the Elekta linac head, and by defining the space phase as a source above the target, the possibility of out-of-field doses in other organs of the body in the treatment of liver cancer using the MCNPX code is examined. The method is described in the section below.

Materials and Methods

The research was done in three steps. First, the phase space of the Elekta linac was calculated using the Geant4 toolkit and MCNPX code. Then, the scattering factor was calculated using the accelerator phase space. Finally, dose evaluation was performed inside MIRD phantom for the treatment of liver cancer, the out-of-field dose in other organs was also calculated. Each of these steps is explained below.

Calculating phase space and scattering factor

Geant4 Toolkit and MCNPX code were used to extract the Elekta compact linear accelerator phase space and calculations related to the total scattering factor (Scp). To extract data related to phase space and Scp, the geometry of the Elekta 6 MeV accelerator head was defined according to Figure 1, in which the accelerator head is composed of the following parts: a) Target: A tungsten cylinder with a radius of 2 mm b) Primary colimator: A tungsten cylinder with 10.2 cm high with a Frustum of a Cone. c) Flattening filter: tungsten cone with a height of 1.75 cm, d) Ionization chamber: thin cylindrical layers of copper, plastic and air with a total thickness of 1.5 cm and a radius of 3 cm [7]. e) Mirror: A Capton cylinder with 45-degree. f) Jaws x and y: tungsten cylindrical shells to adjust the field, and finally g) water phantom with the RK304 detector that is located at 10 cm from the water surface.

To perform the simulation, the electron spectrum of the accelerator source was required. The electron source has a Gaussian distribution with an average energy of 6 MeV and a width at half height equal to 3% of the average energy or standard deviation of 0.07 [8-11].

Figure 1: Head geometry of the compact elekta linear accelerator

An input file was required to run the simulation program. Geometry information and accelerator source were written in the necessary files for simulation by GEANT4 Toolkit and MCNPX code data and prepared for computer execution. The simulation was performed using a 7-core computer with 8 GB of RAM and a 2.9 GHz CPU. By running the code and stopping the electron in target, an X-ray is generated. The X-ray spectrum changes through different parts of accelerator head due to the absorption of photons and the production of secondary photons due to the interaction of photons with matter [12], and its intensity decreases and the spectrum becomes weaker at low energies. X-rays are produced at a   0-0.2 MeV range, which is considered as a photon source. In order to perform dosimetry calculations with high accuracy, the program must be run for several days. To solve this problem, the accelerator phase space must be calculated and defined in GEANT4 Toolkit and MCNPX code files. Phase space contains spatial information [13], energy and angles of the secondary photons passed over target in the accelerator head, which is obtained by running the GEANT4 Toolkit and MCNPX code. After calculating the phase space and by defining the phase space data in the GEANT4 Toolkit and MCNPX code, the Scp at a depth of 10 cm of water phantom for 5×5, 10×10, 20×20, 25×25 and 30×30 cm² field sizes were calculated. Scp is equal to the dose ratio at a depth of 10 cm for each field size to a dose at a depth of 10 cm of the reference field 10 × 10 cm² [14].

Cancer risk (ELCR) and Out-of-field dose calculations in liver cancer radiotherapy

Lifetime cancer risk is calculated by quantifying the risk of cancer (ELCR) using Equation 1 [15].

ELCR = D × RF                                                           (1)

In Equation 1 D(Sv) is the out-of-field dose and RF Risk factor (Sv^-1) indicates the risk of fatal cancer in each organ. For random effects, this value is 0.05 according to ICRP 60 [16-17].

To calculate the out-of-field dose in liver cancer radiotherapy, a MIRD phantom is placed on a patient bed 100 cm below the accelerator head as shown in Figure 2. The phase space is defined as a source above the target. By adjusting the patient bed and field size, the accelerator prepares to irradiate the liver tissue in the MIRD phantom. By executing the code, the amount of dose received in the liver tissue and other organs of the body is calculated in unit of MeV/g per source particle using f6 tally. The following coefficients are considered to convert the f6 tally output to Gy.

1. The probability of producing a photon due to the collision of an electron that is 0.86.
2. The conversion factor of MeV/g to Gy that is 1.6e10.
3. Coefficient of accelerator current

Relationship between accelerator current I(mA), number of electrons (n) and irradiation time t(s) is (I=ne/t). The number of electrons is n = It / e. For t=1 s and I=4 mA, the number of electrons is

4e-3 × 1/1.6e-19 = 2.5e16

4. Coefficient related to irradiation time

Previous coefficients are calculated for the irradiation time as 1 second. If the irradiation time is t seconds, the result must be multiplied by the irradiation time to calculate the dose.

Figure 2: MIRD phantom under the head of the compact model Electra accelerator

Results and Discussion

The results of phase space calculation

By placing a detector cell in different parts of the accelerator head, the characteristics of energy spectrum, angular distribution and spatial distribution of generated and passed photons were calculated. The energy spectrum and angular distribution of the photons produced after the target and the spatial distribution of the photons passed after the flattening filter are shown in Figures 3, 4 and 5.

Figure 3: X-ray spectrum produced in the Compact Elekta accelerator

Figure 4: Angular distribution of X-rays generated in the head of the Compact model accelerator

Figure 5: Spatial distribution of photons on a plane perpendicular to the accelerator axis after flattening filter

According to Figure 4, the angular distribution of the X-rays produced is a Gaussian distribution with standard deviation of 1.33 degrees and a half-height width of 3.12 degrees. According to Figure 5, the spatial distribution of photons emitted from the target in different parts of the accelerator shows that the spatial distribution has a uniform distribution perpendicular to the axis of the accelerator.

Therefore, three characteristics of the energy spectrum, angular distribution and spatial distribution in Figures 3, 4 and 5 can be considered as the phase space characteristics of photons produced by the collision of an electron beam with a target.

Results of scattering factor calculation

By defining the phase space data in the GEANT4 Toolkit and MCNPX code, the Scp at a depth of 10 cm of water phantom for 5 × 5, 10 × 10, 20 × 20, 25 × 25 and 30 × 30 cm² field sizes was calculated. The result was shown that with increasing the field size, the Scp increased, due to the scattering of photons by Jaws and other accelerator components. The results of the Scp calculation by GEANT4 Toolkit and MCNPX code are given in Table 1. Table 2 shows the percentage agreement between experimental and theoretical calculation of Scp by GEANT4 Toolkit and MCNPX code. The results in Table 2 show that there is a 96% and 93% agreement between the MCNPX and GEANT4 simulations, and practical values of Scp, respectively.

Table 1: Results of Experimental and theoretical calculation of Scp by GEANT4 Toolkit and MCNPX code

Table 2: The Compatibility level between practical and MCNPX and GEANT4 simulation value of Scp

Results of out-of-field dose calculation

By applying the phase space characteristics above target in the head of the Elekta accelerator and by adjusting the field as 25×25, 20×20, 10×10, 5×5 and 30 × 30 cm², the out-of-field dose calculation for liver cancer treatment was examined. The results show that the reached dose is significantly outside the above fields due to the scattering of photons. The 5×5 cm² field size was found to be suitable for irradiation to the liver organ.

As can be seen from Figure 6, the dose reaches outside the 5×5 cm² field size, which indicates the scattering of X-ray photons by the accelerator components, the environment and the patient's body. The dose reached the liver and other organs were also calculated using the f6 tally if a 45 Gy dose was injected into the liver to treat cancer. Table 3 shows the results of the dose reached other organs in the treatment of liver cancer with a dose of 45 Gy.

Figure 6: Dose distribution around the liver for 5 ×5 cm² field size: a) depth dose distribution, b) surface dose distribution

Table 3: The dose reached to other organs in the treatment of liver cancer with a dose of 45 Gy

Other= over skeleton; leg bones, arm bone, scapulae, pelvis, rib cage, spine, skull-cranium, facial skeleton

Conclusion

Accelerators are widely used to treat cancer. Compact model Elekta accelerator is one of the types of linear accelerators in Iran that is used for this purpose. Usually, a treatment plan adjusted to the size of tumors in the target tissue is planned for the treatment of cancer. The goal is to deliver the highest dose to the target tissue and the lowest dose to the adjacent tissues. However, due to the scattering of X-ray photons from different parts of the accelerator, the environment and the patient's body tissues adjacent to the target tissue are also at risk. In this paper, we first calculated the scattering factor of photons in different fields. By calculating the scattering factor, the results showed that Scp increases with increasing field size. To calculate the scattering factor, the accelerator phase space data with Geant4 Toolkit and MCNPX code were used. Phase space was used to speed up the calculations. Then, the characteristics of the phase space were assumed as a photon source at the target location of the accelerator head. The case designed for the treatment of 15Gy liver cancer was calculated. The results showed that the pancreas receives a dose greater than 15 Gy due to its proximity to the target tissue and the organs around the liver receive a dose less than 15 due to their distance from the liver. The effect of air on photon scattering and increasing dose and dose leakage was calculated to be 6.69%.  This study was devoted to determining the unwanted dose due to scattered photons to the out-of-field organs and subsequently estimated the risk of secondary cancers in the patients undergoing liver radiotherapy. A Medical Linear Accelerator (Compact Elekta 6 MV) was modeled using the Geant4 Toolkit and MCNPX code to simulate liver radiotherapy with different field size. Dose evaluation was performed inside MIRD phantom. The average photon equivalent dose in out-of-field organs is between 2.1-0.031 Sv Gy⁻¹, for the organs far from the Planning Treatment Volume (Brain) and those close to the treatment field (pancreas). Evidence showed that pancreas with 1.58% and Brain with 0.024% have the highest and lowest risk of secondary cancer, respectively. Accordingly, this study introduced the pancreas as an organ with a high risk of secondary cancer which should be paid more attention in the follow-up of patients undergoing liver radiotherapy. The results of this study showed that radiologists should be more careful in their treatment plan because of the possibility of secondary cancer in the tissues far from the field size.

Acknowledgments

The authors would like to thank all technicians of Shafa Hospital Nuclear Medical Center for providing us technical support required for carrying out this work.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions

Neda Zarei and Dr. Mohammad Reza Rezaie designed and simulated this study and Dr. Ali Jomehzadeh analyzed the data. Neda Zarei proceeded to the data quality control and the manuscript drafting. Mohammad Reza Rezaie revised the final version.

Conflict of Interest

None of the authors has any conflict of interest to declare.

ORCID:

Neda Zarei

https://www.orcid.org/0000-0003-2090-5926

Mohammad Reza Rezaie

https://www.orcid.org/0000-0002-2183-7729

Ali Jomehzadeh

https://www.orcid.org/0000-0002-6729-2891

HOW TO CITE THIS ARTICLE

Neda Zarei, Mohammad Reza Rezaie, Ali Jomehzadeh. Radiation Hazards Investigation of Photon Scattering by Elekta 6 MeV Linac during Liver Cancer Treatment. Chem. Methodol., 2022, 6(8) 582-590

https://doi.org/10.22034/CHEMM.2022.338957.1493

URL: http://www.chemmethod.com/article_150334.html