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To develop an assumption-free methodology for testing geometry of linacs.The problem of projecting a fiducial positioned in a predefined point in a 3D space and attached rigidly to a treatment table of a radiotherapeutic device onto an imaging plane with unknown characteristics from a source with unknown coordinates is formulated. The problem of determining these unknowns is formulated as an optimization problem. The problem of determining the gantry/the collimator rotation axis and angle from projection of additional fiducials is also formulated and solved. Analytical methodology is developed for determining isocenter position and an error of estimating isocenter position. The developed methodology is tested in simulations.Very good agreement between preset and calculated values of quantities of interest was found in the simulations. In all cases, the proposed schemes enabled determination of the geometric characteristics of a radiotherapeutic device with accuracy better than one hundredth of a millimeter and one hundredth of a degree.A concept of a multimodule multifiducial phantom has been introduced. Analytical framework has been developed to extract geometric characteristics of radiotherapy devices from projection images of a phantom. The phantom design and the methodology developed have been tested in simulations.