Monte Carlo is a highly accurate method for radiotherapy in areas as diverse as treatment head design, machine calibration, and image-guided radiotherapy. For example, recent developments in precision radiotherapy require higher accuracy dose calculation to realize full clinical benefit. Monte Carlo can meet this demand. Unfortunately, Monte Carlo is extremely difficult to implement. The objective of this study is to remove this obstacle to widespread clinical use of Monte Carlo. The key requirement is an accurate, easily commissioned model of the treatment beam. Commissioning is the process of determining the parameters of the model that match the measurements peculiar to that beam. Using current methodology, a clinical physicist unfamiliar with the method would take months to commission a standard 8-beam accelerator. This is unacceptably long. The proposal is to prove beam models may be both accurate and easy to commission. This will be accomplished with 3 Specific Aims: 1. Prove Monte Carlo is sufficiently accurate for validating beam models (1%) by testing the accuracy for simulating the key accelerator components (target and scattering foils), using measurements on a research accelerator, unique in providing beams with accurate energies. 2. Combine experiment with Monte Carlo to establish detailed maps of particle fluence (particle number, energy, direction) of unprecedented accuracy (2%) for clinical beams. Detailed measurements on accelerators from the 3 major manufacturers, including source and geometry variations and beam asymmetry, will be converted to fluence maps using Monte Carlo. 3. Develop beam models with parameters extracted from routine measurements, with commissioning done in under a day per beam. Accuracy will be evaluated using the fluence maps and dose measurements. The practical result will be a pair of beam models, one for electrons, one for x-rays, which are easy to commission, meet the stringent accuracy requirement of modern radiotherapy, and apply to the bulk of radiotherapy units in clinics today. This will allow clinics around the country to take advantage of the computational accuracy of this state-of-the-art clinical tool. [unreadable] [unreadable]