Proton stereotactic radiotherapy shares fundamental principles with general proton therapy physics, specifically range uncertainties and broad beam measurement techniques. Significant differences emerge when treating with smaller field sizes that suffer lateral disequilibrium and when fractions are reduced. This session will explore the history and scope of proton stereotactic radiotherapy in clinical practice. Uncertainties and treatment planning methods specific to stereotactic treatments will be discussed. The session will include an overview of the physical properties of small proton fields and resulting needs for accurate measurements and modeling of dose distributions for radiosurgery treatment planning. LEARNING OBJECTIVES 1. Understand the clinical rationale for proton radiosurgery. 2. Understand the similarities and differences from general proton therapy. 3. Understand the similarities and differences from photon stereotactic radiosurgery. 4. Understand the basic physics and clinical physics methods for measuring and commissioning a radiosurgery program.