Project Summary Despite advances in surgery, radiation and chemotherapy, the treatment of malignant brain tumors remains one of the most perplexing clinical areas with the fewest advances in treatment in the field of oncology. It is of central importance to determine treatment response or failure, for both patients and physicians. Currently, MRI is the most commonly used method to assess treatment response in patients. However, it has its shortcomings as often tumor recurrence and treatment effect cannot be differentiated. Other potential functional methods for response assessment include PET, PET-CT, magnetic resonance spectroscopy, and dynamic contrast enhanced MRI, however, each technique has limitations. Diffusion-weighted MRI (DWI) provides information on tissue microstructure that is not obtainable by other means. Although DWI has shown some promise in its ability to predict early radiation response in brain tumors, they are fundamentally limited in terms of what cellular characteristics they are able to report. The overall goals of this proposal are to develop and validate a novel voxel-based subset of DWI, termed as quantitative Temporal Diffusion Spectroscopy (qTDS), for quantifying specific cellular properties (notably, cell size and cell density) in brain tumors. Such cellular properties could serve as improved early indicators of radiotherapy response. For example, apoptosis and mitotic catastrophe have been reported to be the main forms of radiation-induced cell death. Although mitotic catastrophe tends to increase cell size, most of cells undergoing mitotic catastrophe die via apoptosis. Thus, qTDS is expected to be able to detect the cell shrinkage associating with apoptosis. Recently, we have shown that qTDS is feasible on clinical scanners. However, the mechanism of qTDS and its role in radiation response assessment needs further investigation and validation. Therefore, in this study we will pursue the following two Specific Aims: [1] To develop, optimize, and validate qTDS for voxel-based quantification of cell size and cell density and assessing radiation-induced cellular responses in vitro in different types of human glioblastoma cells and in vivo in a human glioblastoma xenograft model; [2] To evaluate and validate qTDS for early detection and characterization of radiotherapy response in a 9L gliosarcoma model, and a human glioma primary orthotopic xenograft model. Impact: The methods investigated in this proposal will provide the cancer community with imaging methods that characterize the underlying biology of brain tumors and predict radiation treatment response for brain tumors. These findings can then be incorporated into clinical trials and ultimately, become standard-of-care practice.