Design of a spherical applicator for intraoperative radiotherapy with a linear accelerator-a Monte Carlo simulation

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 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 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 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 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 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 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. • For electron beams of energies 4, 6, 9 and 12 MeV, the dose rate was about 30, 40, 50 and 60 cGy min^-1, 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.