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Design of a Spherical Applicator for Intraoperative Radiotherapy with a Linear Accelerator—A Monte Carlo Simulation

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Design of a Spherical Applicator for Intraoperative Radiotherapy with a Linear Accelerator—A Monte Carlo Simulation Physics in Medicine & Biology PAPER Design of a spherical applicator for intraoperative radiotherapy with a linear accelerator—a Monte Carlo simulation To cite this article: P Ma et al 2019 Phys. Med. Biol. 64 015014 View the article online for updates and enhancements. This content was downloaded from IP address 124.17.113.55 on 07/01/2019 at 06:26 IOP Phys. Med. Biol. 64 (2019) 015014 (12pp) https://doi.org/10.1088/1361-6560/aaec59 Physics in Medicine & Biology Phys. Med. Biol. 64 PAPER 2019 Design of a spherical applicator for intraoperative radiotherapy 2018 Institute of Physics and Engineering in Medicine RECEIVED © 2 August 2018 with a linear accelerator—a Monte Carlo simulation REVISED 8 October 2018 PHMBA7 P Ma1, Y Li2, Y Tian1, B Liu3, F Zhou3 and J Dai1 ACCEPTED FOR PUBLICATION 29 October 2018 1 Department of Radiation Oncology, National Cancer Center/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, People s Republic of China 015014 PUBLISHED ’ 21 December 2018 2 Department of Radiation Oncology, Sun Yat-Sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center for Cancer Medicine, Guangdong, People’s Republic of China 3 Image Processing Center, Beihang University, Beijing, People s Republic of China P Ma et al ’ E-mail: [email protected] Keywords: spherical applicator, intraoperative electron beam radiotherapy, roundness Printed in the UK Abstract Currently only flat dose distributions can be generated by electron beams of a linear accelerator for intraoperative radiotherapy (IORT). However, spherical dose distributions are more desirable for PMB certain types of cancers such as breast cancer and brain cancers. In this study, we propose the design of a spherical applicator for delivery of spherical dose distributions. The spherical applicator consists 10.1088/1361-6560/aaec59 of an upper cylindrical collimator to collimate the electron beam, a middle scattering foil to scatter the beam and a lower hollow sphere with a modulator to shape the beam and a spherical shell used 1 to contain the modulator. Monte Carlo (MC) codes EGSnrc/BEAMnrc and EGS4/DOSXYZ were employed to model the head of the Mobetron, the spherical applicator, and to calculate the dose distributions. Apart from the scattering foil made of tungsten, the region between the scattering foil 1361-6560 and the inner surface of the modulator is empty whereas the remainder of the spherical applicator made of soft tissue-equivalent materials such as PMMA. As a measure of how close an object 1 approaches a perfect circle, roundness was introduced to evaluate the dose distributions. In addition, the dose rate after modulation was investigated. A spherical applicator with a 20 mm-diametre cylindrical collimator and a 50 mm-diametre hollow sphere was designed. For electron beams of 12 energies 4, 6, 9 and 12 MeV, the foil thickness was set to 0.3, 0.5, 0.7 and 1.2 mm, and the dose rate was about 30, 40, 50 and 60 cGy min−1, respectively. The roundness of the isodose curves in the coronal plane through the centre of the spherical applicator ranged from 0.01 to 0.12 cm whereas that in the axial plane ranged from 0.05 to 1.38 cm. Experiments are planned to further evaluate the feasibility of this spherical applicator design. Highlights • A spherical applicator with a 20 mm-diametre cylindrical collimator and a 50 mm-diametre hollow sphere was designed for delivery of spherical dose distributions for IORT with a linear accelerator. 1 • For electron beams of energies 4, 6, 9 and 12 MeV, the dose rate was about 30, 40, 50 and 60 cGy min− , respectively. • As a measure of how close an object approaches a perfect circle, roundness was introduced to evaluate the spherical dose distributions. • Experiments are planned to further evaluate the feasibility of this spherical applicator design. 1. Introduction Intraoperative electron beam radiotherapy (IOERT) is a treatment involving the delivery of a high-dose electron- 21 beam radiation during operation onto a target area while sparing the surrounding tissue. The advantage of IOERT is the direct identification and irradiation of the surgically opened tumor bed while protecting healthy December © 2018 Institute of Physics and Engineering in Medicine 2018 Phys. Med. Biol. 64 (2019) 015014 (12pp) P Ma et al tissues in front of the target as well as those behind the target by applying the appropriate electron energy (Beddar et al 2006). IOERT can be implemented either with a conventional linear accelerator by transporting the patient inside the radiation therapy room or a dedicated mobile accelerator able to perform IOERT directly inside the operating room (Strigari et al 2004, Jaradat and Biggs 2008). For linear accelerators, the available dose distributions have only flat dose profiles. The idea of creating an applicator that changes the flat geometric shape of the IOERT dose distributions and allows the accelerator to deliver spherical dose distributions comes from a clinical question formulated by the surgical team of our insti- tute regarding the treatment of breast cancer with linear accelerator. The high-dose-rate system Mobetron (IntraOp Medical Corporation, Sunnyvale, USA) is designed as a mobile device to be installed in an existing operating room. This allows moving the accelerator to the surgically opened patient. The Mobetron provides high dose-per-pulse electron beams with energies 4, 6, 9 and 12 MeV. To produce different field sizes, the Mobetron is equipped with a set of cylindrical and rectangular applicators. The aim of this work was to perform Monte Carlo (MC) simulations to design a spherical applicator for the Mobetron. The MC codes EGSnrc/BEAMnrc and EGS4/DOSXYZ were employed to model the head of the Mobetron and to calculate dose calculations. A 50 mm-diametre spherical applicator was simulated with a single entry port for electron beams of energies 4, 6, 9 and 12 MeV. The design objectives were: (1) to produce spherical dose distributions and (2) to provide an adequate dose rate to minimize the treatment time to a clinically accept- able level. 2. Materials and methods 2.1. Design of spherical applicator 2.1.1. Key aspects of the design The spherical applicator consists of three sections: (i) an upper cylindrical collimator (inner diametre 20 mm and thickness 5 mm) to collimate the electron beam, (ii) a middle scattering foil (diametre 23 mm) to scatter the beam and (iii) a lower hollow sphere (outer diametre 50 mm) with a modulator used to modulate the beam and a spherical shell used to contain the modulator. The design process (figure 1) involves nine steps. The first three steps are to determine the scattering foil, and the remaining six steps are to determine the modulator. First, the thickness of the scattering foil is determined by setting a series of scattering foils of various thicknesses (a range from 0.1 to 1.5 mm in steps of 0.1 mm) and inves- tigating their effect on the angular spread of the electron beam and dose rate (detailed in section 2.1.2). Second, the modulator is designed by creating a set of layers that were used to obtain the dose-radial thickness relation- ship curve. For that purpose, the minimum dose on the outer surface of the hollow sphere for spherical shell with zero radial thickness was calculated and designated the objective dose. The angular distribution of radial thick- ness to deposit the objective dose is assigned by interpolation, and the geometry of the modulator is ascertained (detailed in section 2.1.3). 2.1.2. Design of scattering foil When a beam of electrons passes through a medium, the electrons undergo multiple scattering arising from Coulomb force interactions between the incident electrons and, predominantly, the nuclei of the medium. As a result, the electrons acquire velocity components and displacements transverse to their original direction of motion. The International Commission on Radiation Units and Measurements (ICRU 1980) defines the mean- square scattering angle, which varies approximately as the square of the atomic number and the thickness of the medium. In this study, to obtain the spherical dose distributions, the scattering angle must be sufficiently large. Therefore, high-Z material tungsten, the target material in conventional x-ray tubes, was used as scattering foil, the thickness of which was designed to maintain an adequate dose rate. The scattering foil is a 23 mm-diametre plate located between the collimator and the hollow sphere (figure 2). To determine the thickness of the scattering foil, a set of tungsten foils of various thicknesses (ranging from 0.1 to 1.5 mm in steps of 0.1 mm) were designed and their effect on the angular spread of the electron beam was investi- gated. Moreover, the irradiation time was determined by the dose rate that could be decreased through modula- tion. It has been reported that the lowest dose rate to deliver partial-breast irradiation is 45 cGy min−1 using a x-ray source (Sethi et al 2018).Therefore, the dose rate after the interaction with the scattering foil was estimated and controlled to be higher than 70 cGy min−1 to make dose rate remain at the same level after the attenuation added by a modulator. 2.1.3. Design of modulator The modulator is embedded in the hollow sphere, and its outer surface fits to the inner surface of the spherical shell. To describe the inner surface feature of the modulator, the radial thickness was defined as the thickness of the modulator along the radial direction. 2 Phys. Med. Biol. 64 (2019) 015014 (12pp) P Ma et al Figure 1. Flow chart of the spherical applicator design process.
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