Imaging Radiation-Induced Biological Response: Mechanisms of Tumor Control and Normal Tissue Remodeling

Abstract

Radiation therapy is a cornerstone in cancer treatment, however, the high doses required to achieve tumor control often result in injury to surrounding healthy tissue. Despite advances in treatment conformity, the therapeutic index is still limited by the combined effects of low-dose radiation spread and the delivery of high radiation doses to tissues in close proximity to the planned target volume, especially when tumors are near critical organs. These limitations highlight the need for methods that improve tumor control without increasing toxicity to surrounding tissue, while also enabling characterization of the effects of high radiation doses on normal tissue. This study characterized the in vivo response to radiation by evaluating normal tissue damage, tumor suppression, and associated immune activation using multimodal imaging. Ultrasound, computed tomography (CT), and fluorescence imaging were implemented in two murine models to provide longitudinal characterization of treatment response. In an E0771 breast cancer model, caged gold nanostars (CGNS) were investigated as radiosensitizers in combination with external beam radiation therapy and anti-programmed death ligand (aPD-L1) immunotherapy to reduce the radiation dose necessary for tumor control. Ultrasound imaging provided greater analysis of tumor volume by providing three-dimensional information and improving accuracy compared to caliper measurements. Additionally, shear wave elastography provided early prediction of treatment response, where tumors that progressed toward suppression exhibited lower stiffness as early as 6 days post-treatment. Flow cytometry highlighted differences in immune cell populations across treatment groups, further explaining treatment outcomes. Analysis of lymphocyte balance, myeloid polarization, and differentiation flux indicated that a balanced but active immune response was associated with improved tumor suppression. The combination of CGNS, radiation therapy, and aPD-L1 resulted in the greatest and most sustained tumor growth inhibition, achieving 42% inhibition on day, the greatest among treatment groups. Through a liver irradiation model, longitudinal imaging was used to quantify radiation-induced fibrosis. Ultrasound, CT, and fluorescence imaging did not detect fibrosis over the course of the study, consistent with histological findings revealing that only the 50 Gy group exhibited mild fibrosis, which remained below detectability of the imaging techniques. These results suggest that radiation induced fibrosis is dependent on both dose and irradiated volume, as well as biological factors influencing tissue response. Although significant fibrosis did not develop, these results demonstrate the potential of multimodal imaging to assess tissue remodeling following radiation exposure. This work provides experimental confirmation of the role imaging plays in characterizing treatment outcomes and provides strategies for maintaining tumor control while minimizing damage to healthy tissue.