Our overall goal is to complete development of a two-dimensional (2D) planar dosimetry system for brachytherapy dose measurement based upon a promising new detector system, plastic scintillator (PS) sheet. Solid organic scintillators convert energy absorbed from ionizing radiation fields into blue fluorescent photons, creating a measurable optical signal related to absorbed dose in the surrounding medium. PS is a highly sensitive detector and, unlike current point-by-point measurement techniques, supports high resolution dose measurements throughout an entire plane with online signal acquisition times on the order of a few minutes. During our first funded period, we successfully developed new highly-efficient PS systems with well localized optical responses to ionizing radiation which can be loaded with medium atomic number additives to closely match the radiological properties of water. However, despite water-equivalent composition, PS response varies by more than 30% over the 20 to 400 keV photon energy range. This nonlinear response, an inherent property of the underlying scintillation process, is not explained by current theories and is a new and fundamental observation. We have successfully developed a 2D prototype dosimetry system using a liquid-nitrogen cooled digital camera. Noise analysis demonstrates that high precision (+/- 2%) measurement around low dose-rate sources, including I-125 seeds, is possible with technically feasible improvements in light production and collection efficiency. In specific aim 1, we propose to investigate the mechanistic basis of PS sensitivity dependence on electron energy and use this understanding to develop maximally sensitive dosimeters with flat energy response to 20 KeV-1 MeV photons. Electron energy loss spectroscopy and discrete-collision Monte Carlo simulations of electron track structure will be used to characterize the dependence of PS primary excitation processes on energy. Compton-coincidence techniques will be used to precisely measure PS sensitivity energy dependence, and to study the role of secondary photophysical processes in modulating nonlinear energy response. In specific aim 2, we will optimize design of the 2D dosimetry system, characterize its imaging system performance, and develop image processing algorithms to eliminate spatial nonuniformity, high frequency noise and image blurring due to scattering and transport of light. We intend to increase overall system sensitivity 100-500 fold, so that 3D dose distributions of intravascular, high dose-rate and low dose-rate brachytherapy sources can be measured with an accuracy of 3%, variable spatial resolution of 0.2 to 1 mm, and acquisition times per plane of a few minutes. Finally, Specific Aim 3 seeks to demonstrate system utility and accuracy by measuring 3-D dose-rate distributions about brachytherapy sources in clinically-relevant homogeneous and heterogeneous geometries.