Developing radiotherapy radiation plan requires detailed knowledge of the physical process of ionizing radiation energy transfer on matter, variety of algorithms that allow sufficiently accurate and acceptable fast calculation. Influence of positioning errors on dose calculation for adjuvant radiation of left breast is studied in this graduation work. Comparison of the quality of the plan was made by comparing the radiation dose-volume histograms (DVH). The volume of the patient, or the geometry in which calculation are made, is given in the form of CT slices thickness of 2 mm. Oncologist on slices marks the contours of the target volume, organs at risk and sets the conditions in the form of fractionated therapeutic dose to the target volume (PTV, CTV) and dose restrictions. The whole volume is divided into spatial elements (voxels), cubes with edge of 2 mm. Average electron density for each cube is known and to which tissue or plan marked volume it belongs. Physicist makes a plan of radiation, directing selected therapeutic beams so that the therapeutic volume reaches default homogeneous absorbed dose and other tissues within the allowable limits. Multigrid superposition algorithm it is applied. The calculation will take into account the different electron density of individual volume elements. However, in the process of therapy, there is another uncertainty - an error that occurs during patient positioning for radiation (set-up error), which can be systematic or random and can be in range from a few mm to a few cm in all directions of coordinate axis. This work analyze the impact of such errors on dose distribution. Setting error is simulated by moving the isocenter in the three dimensions by 2.5 mm at an interval of -30 mm to +30 mm. That generated 72 radiation plans and the process was repeated for 5 patients. To extract and process the data needed to compare radiation plans, programming language Python has been used. Preparations for a class on the subject of the photoelectric effect, which is taught in high school, is developed in the methodological part.