dissertationThe concept of relative biological effectiveness (RBE) in radiation therapy and diagnostic imaging for a particular radiation type is defined by the ratio of absorbed dose of a reference radiation, typically a low linear energy transfer (LET) radiation to the absorbed dose of a test radiation of typically higher LET that achieves the same biological effect. It is used to quantify and compare expected outcomes (therapy) and deterministic and stochastic risk (imaging, radiation protection) from different types of ionizing radiation. Numerical modeling of RBE and other metrics related to the biological response to ionizing radiation in radiation therapy, diagnostic X-ray imaging, and other related fields is becoming increasingly important as hadron therapy becomes more prevalent and as the concerns associated with diagnostic X-ray dose increase. This dissertation develops and tests a multiscale biophysical model to aid in both estimating clinically relevant biological metrics and to further understand the underlying mechanisms in the special cases examined. The investigation of these special, asymptotic cases in clinical applications of ionizing radiation are used to further refine and improve the multiscale model. On the therapy side, the high-LET binary radiation therapy of boron neutron capture therapy (BNCT) is used to test the multiscale model. In this therapy modality, there is both dependence on the primary neutron source and biodistribution of the boron with respect to the targeted cells. The radiobiology of these densely ionizing, short range particles are much different than that of sparsely ionizing photons. On the diagnostic imaging side, the RBE and dosimetric characteristics of computed tomography (CT) are examined with the multiscale model, looking specifically at effects of iodine enhancement. Recent experimental data showing that kV X-rays and electrons have an RBE greater than unity are in line with predictions from the multiscale model. Furthermore, the reported studies also provide strong support for the hypothesis that the RBE for DSB induction is within a few percent of the RBE for cell survival over a wide range of photon and electron energies. The final part of this research focuses on the further integration and expansion of the multiscale model
