Modeling the head of PRIMUS linear accelerator for electron‐mode at 10 MeV for different applicators

Abstract Objective This study is to validate the utilization of Monte Carlo (MC) simulation to model the head of Primus linear accelerator, thereafter, using it to estimate the energy fluence distribution (EFD), the percentage depth dose (PDD), and beam profiles. Materials and Methods The BEAMNRC code that is based on the EGSNRC code has been used for modeling the linear accelerator head for 10 MeV electron beam with different applicator sizes (10 × 10, 15 × 15, and 20 × 20 cm2). The phase space was acquired from BEAMNRC at the end of each applicator and then used as an input file to DOSXYZNRC and BEAMDP to calculate the EFD, PDD, and beam profiles. Results There were a good consistency between the outcomes of the MC simulation and measured PDD and off‐axis dose profiles that performed in a water phantom for all applicators. The PDD for the applicators proved to be favorable as a direct comparison of R100, R90, R80, and R50 yielded results of < 2 mm, while it was 6 mm in R100 for the applicator 15 × 15 cm2. The discrepancies in the surface doses (<3%) showed a quick decline in the build‐up region and differences reached 0% within the first 2.4 mm. For the beam profiles comparison, the differences ranged from 2% (2 mm) to 3% (6 mm) for all applicators. Conclusion Our examination demonstrated that the MC simulation by BEAMNRC code was accurate in modeling the Primus linear accelerator head.


| INTRODUCTION
The fundamental modalities of malignancy treatment are radiotherapy, chemotherapy, and surgery. 1 The treatment modality is usually chosen based on the stage and type of disease. Over 40% of all cancer sufferers are treated with radiation treatment whereby a therapeutic dose of ionizing radiation is conveyed to a malignancy site in the expectation of killing tumor cells. The objective of radiation treatment is to kill tumor cells by causing irreparable damage to their DNA while sparing normal cells as meager harm as possible. 2 There are several machines in use for radiotherapy cancer treatment, yet linear accelerator (LINAC) based radiotherapy is the most common used machine worldwide. Deep-seated tumors are usually treated by x-rays produced by bremsstrahlung interaction of electron beam with a target. However, superficial tumors are usually treated by electron mode of a LINAC. 3 While the limited scope of electrons in tissue has restorative advantages in radiation treatment including electron beams, prediction of dose for electron beams incident on heterogeneous tissue can be challenging in radiation treatment plannin. 4 A uniform 'plateau' of dose could be delivered by a single electron beam, ranging from 90% to 100% of maximum central axis dose, in which the dose suddenly falling off both laterally and distally. This has allowed superficial cancers and disease within 6 cm of the patient's surface to be irradiated with low dose to underlying normal tissues and structures, something usually not possible with x-ray therapy. 5 Electron beams have been successfully used in numerous sites such as head and neck to avoid irradiation for spinal cord. It is also used for chest wall radiotherapy to avoid excessive irradiation of lung. 6 The complex nature of electron tissue interactions means that electron beams are generally difficult to model. In electron beam therapy, calculation of collimator scatters and leakage, prediction of dose in small fields, situations involving sudden changes in surface contours, small inhomogeneities, and oblique beam incidences are particularly challenging. 7 Monte Carlo (MC) simulation is a precise and specified method of modeling the complex electron source configurations and geometries used in radiation therapy. It is known to be very accurate when used properly for patient-specific dose calculations. 8,9 Monte Carlo simulation can give an extensive variety of accurate data, including data which is difficult or impossible to quantify. 10 A portion of the early employment of the MC method included estimations of mass stopping power ratios and the relationship between mean energy at the phantom surface and the practical range of the electron beam as recommended for electron beam dosimetry by ICRU Report 35. 11,12 Monte Carlo can possibly unravel a significant number of electron transport problems, especially in-patient heterogeneities, encountered with conventional treatment planning algorithms. 13 The principal disadvantage of MC simulation as applied to radiation transport has been the long computation time. For example, the electron beam range (R 50 ) in water is highly sensitive to the initial electron energy (0.1 cm change per 0.2 MeV) and the source energy is, therefore, the primary tuning parameter in electron beam simulations. However, electron beams are also very sensitive to all components in the beam path and therefore accurate geometric descriptions of all treatment head components is required. 18 Monte Carlo simulations of radiation treatment machine heads provide practical means for obtaining energy spectra and angular distributions of photons and electrons. So far, most of the work published in the literature has been limited to photons and the contaminant electrons knocked out by photons. 19 The dimensions and materials used in various components in the machine head (e.g., primary collimator, flattening filter, etc.) are specified as input to the code. Therefore, a different accelerator can easily be described by modifying these inputs. 20 To confirm the validity of the energy spectra and angular distributions generated by the MC programs, one may calculate dose distributions using these data, and compare the results of calculations with measured depth dose data. 21 We aimed in this study to simulate the electron mode of Sie-

2.B | Experimental measurements
For relative experimental data such as PDD and off-axis profiles, two Step Transport Algorithm) is introduced into the EGS code system to improve the accuracy of modeling of electron transport. 25 We picked EXACT boundary crossing algorithm (BCA) with the goal that electrons are transported in single elastic scattering mode as soon as they reach a distance from the boundary defined by the skin depth for BCA. The default value of three mean free paths is recommended to give peak efficiency.    The phase-space that acquired by BEAM NRC was T A B L E 3 DOSXYZ NRC input parameters that are implemented in our work | 137 used as an information document to BEAMDP to derive energy-fluence distribution. Table 4   (the depth at which the dose reaches 80% of the maximum dose), and R 50 (the depth at which the dose reach to 50% of the maximum dose) of the measured and MC calculated PPD. Figure 5 represents the MC calculation and measured dose profiles of 10 MeV nominalenergy of LINAC at R 100 . In addition, Table 6 presents the R W50 , F r ,   The differences between lateral field size at the 50% dose level (R W50 ), Penumbra widths, P 90−10 and P 80−20 are summarized in Table 6, which obtained using both calculated and measured data.

Incident particle All
The differences between the measurements and the simulations result in lateral field size at the 50% dose level (R W50 ) were found to be <2 mm.  | 139 good matching between the simulated and measured data has been obtained.

ACKNOWLEDGMENT
The authors sincerely thank Prof. Dr. Abdelhady El-Kamel the head of nuclear physics group at Faculty of Science, Assiut University for his kind support during measuring, analyzing, and writing this research work.

CONF LICT OF I NTEREST
The authors declare no conflict of interest.