Microdosimetric characteristics of 50 kV X rays at different depths for breast intraoperative radiotherapy

An intraoperative radiation therapy (IORT) device with 50 kV X rays was designed to deliver a single dose to the tumour bed after local excision of breast cancer. The quality of a radiation can be determined by the microscopic distribution of energy transfers along and across the charged particle tracks. The lineal energy, y, serves as an accurate measure of local energy concentration. The dose mean lineal energy, y_D, is an indicator of radiation quality. For low linear energy transfer radiation, the ratio of its dose mean lineal energy to that of ⁶⁰Co gamma rays can serve as a good indicator of the relative biological effectiveness (RBE) at low doses. In this study, microdosimetric simulations are performed for soft tissue irradiated by 50 kV X rays generated from the IORT device, with a 4-cm breast applicator attached. All energy transfers are recorded with the location coordinates in the tissue. Microdosimetric single events in a sphere of 1 μm in diameter are scored as a function of radial distances from the applicator surface. Single-event spectra are then constructed. From those single-event spectra, dose mean lineal energy is calculated. Compared with dose mean lineal energy of ⁶⁰Co gamma rays, the estimated RBEs at low doses are given for the X rays at different depths in the tissue. The RBEs at clinically relevant doses, as a function of depth, are also presented.