The document discusses quality assurance testing for a Varian 21EX linear accelerator used for radiation therapy. It describes various tests performed on the linear accelerator including absolute dose calibration using an ionization chamber and water phantom, measurements of tissue maximum ratios and beam transmission factors, and in-air measurements of collimator scatter factors and output factors. Mechanical tests are also described to check components like light and crosshair alignment, optical distance indicators, readout devices, and axes of rotation. Safety tests include checks of communication devices, beam termination systems, motion controls and limits, dosimetry systems, and backup safety mechanisms. Documentation and analysis of results against tolerance limits is emphasized.