Radiobiological aspects of intraoperative tumour-bed irradiation with low-energy X-rays (LEX-IORT)
Intraoperative radiotherapy (IORT) with a single high dose of low-energy X-rays (LEX) represents a major departure from conventional adjuvant radiotherapy with high-energy photons given in small daily fractions. Thus differences in beam energy, dose distribution, fraction size, dose delivery time, and time of application, may influence the biological effect of the treatment. This review presents the physical and radiobiological properties of LEX, the role of dose delivery time and fractionation, and the relative biologic effectiveness (RBE) of 50 kV X-rays from the Intrabeam machine as used for intraoperative tumour-bed irradiation. Furthermore, evidence for additional biological effects of high single doses such as non-targeted cellular effects, vascular effects, immunological effects, and cytokines, is discussed. Finally, examples of radiobiological modelling of clinical effects are given. In conclusion, the increased RBE, a small volume of high-dose irradiation, and special biological effects of high doses may contribute to the efficacy of intraoperative tumour-bed irradiation with LEX.