Significance/Innovation: Utilizing non-coplanar beams in radiotherapy can significantly improve dosimetry for better normal organ sparing and tumor targeting. However, the optimality of such therapy requires extensive and efficient utilization of non-coplanar beams that are incompatible with existing radiotherapy platforms. The widely available C-arm gantry systems are collision prone, cumbersome, and require undesirable patient couch motion to achieve the non-coplanar angles. The existing robotic system lacks posterior beams and a planning system to integrally optimize beam orientation and fluence, making it extremely limited in achievable dosimetry, target sizes, applicable disease sites, and throughput. Motivated by these clinical and market needs UCLA and RadiaBeam are developing a new robotic radiotherapy platform called Polaris that overcomes the limitations of existing radiotherapy systems, with the following innovative advantages: 1. A super compact 6MV linear accelerator enables unobstructed access to 4π solid angle, including posterior beams. 2. A mathematical framework for integrated beam orientation, source-to-tumor distance, and fluence map optimization, which can efficiently create substantially better plans than the clinical state-of-the-art. 3. A 3D image guidance system will be implemented for the first time on a robotic radiotherapy system without impeding the degrees of freedom in non-coplanar delivery. 4. The combination of hardware and software allows superior dosimetry to be efficiently delivered to targets of all sizes, making our system a superior replacement of all C-arm gantry systems and the existing robotic radiotherapy system as a general-purpose radiotherapy machine. During the SBIR project (NIH Phase II 5R44CA183390-04), Celestial Oncology Inc. was founded to bring the Polaris system to market, and received a $6M series A investment in 2020. This NIH SBIR Phase IIB bridge project will accelerate the completion of the last technical, regulatory, and clinical steps required to bring this life-saving therapy to the clinic. We propose the following aims: Aims: 1a: Mechanically synchronize the X-ray source and detector robots. 1b. Collect projections for circular and helical trajectories for CBCT reconstruction. 1c. Integrate the imaging component for end-to-end IGRT accuracy validation. 2a. Complete the design, manufacturing, verification, and validation of the clinical system. 2b. Submit for US market clearance. 2c. Establish and certify a Quality Management System. 3a. Installation and acceptance tests of the 4π radiotherapy system at UCLA. 3b. Early phase clinical study. Impact: Polaris will serve as a superior replacement of all general-purpose machines with advantages in dose conformity, automation, and throughput. These advantages will drive rapid adoption. The success of this project will clear the last hurdles between the current level of technical development and what is needed to be commercially and clinically...