Dosimetric pitfalls in the application of shielding disk in breast intraoperative electron radiation therapy

Introduction: Intraoperative electron radiotherapy (IOERT) is an IORT technique in which electron beams with different nominal energies are used for irradiation of microscopic residual of tumor bed after surgery. Application of a shielding disk, which is positioned under the target volume, for protecting the normal tissues such as pectoral muscle, lung, and heart is one of the advantages of IOERT in the breast cancer treatment. A common characteristic of these disks is double-layer format; a high atomic number (Z) layer with the ability to absorb the electron beam to prevent penetration to deeper tissues and a Low-Z layer on top of the former to minimize the backscattered electrons from the lower layer. In this study, we have evaluated the misalignment effects of shielding disk on dose distribution inside the target volume and surrounding healthy tissues through EGSnrc MC Simulation.   Materials and Methods: LIAC 12 (Sordina IORT Technologies, S.p.A) is a dedicated IOERT machine that can produce electron beam energies of 6, 8, 10, 12MeV. This machine and its dedicated shielding disks which is made from 3 mm steel and 3 mm PMMA were used in this study. Machine head and employed radio-protecting disk were modeled using the BEAMnrc MC code. Some scoring planes were defined at the bottom of the applicator. Before using the phase-space files, the developed MC models were validated through comparing the Monte Carlo calculated PDDs and TDPS with those measured by ionometric dosimetry. EGSnrc C++ class library was used for simulation and calculation of the dose distribution around the shielding disk in some incorrect positions. Backscatter factor (BSF), shielding factor (SF), and percentage of leakage dose (PLD) was calculated as interested shielding parameters. In addition, some extracted parameters form differential DVH such as V100 and S-Index (Uniformity Index) were used to evaluate the dose distribution in the target.   Results: The BSFs for all correct and incorrect setups at different nominal energies varied from 1.03 to 1.11, while experimental BSF values ranged from 1.02 to 1.1. By increasing the degree of disk rotation and beam energy, the SF decreases, while the PLD would be increased. By increasing the disk rotation, V100 decrements while the S-index is increased inside the target area.   Conclusion: The analysis of uncertainties in shielding disk setup showed that the increment of disk rotation relative to applicator central axis can considerably influence the dose uncertainty inside the target volume and received dose to the organ at risks. Therefore, evaluating the treatment setup after positioning the disk and applicator through intraoperative C-arm imaging can be considered as a mandatory issue for IOERT quality assurance.