Introduction: The brain is a complex organ that coordinates the activity of all functions, but in the occurrence of cancer, these have not remained intact. Considering what cells brain tumors are made of, we distinguish between primary and secondary brain tumors. An important part of medical treatment is radiation therapy, which uses megavoltage photon beams, three-dimensional radiation techniques, and the image-guided radiation therapy system. The size of the margins used in radiotherapy of the brain is up to 2 cm. Purpose: The purpose of this thesis was to evaluate whether the excising margin of the clinical tumor volume and planning target volume correspond with calculated radiation margin based on systematic errors and to define radiation margins required for irradiation of individual brain lobes. Methods: A retrospective cross-sectional study was performed at the Institute of Oncology Ljubljana. We chose a period of one year, from September 1, 2018 to September 1, 2019. In the patient's electronic/physical irradiation maps, we checked the systematic errors and calculated their average and the size of radiation margins, both divided by brain lobe. We compared them with the existing ones. The data were statistically analyzed using a statistical data processing program. Results: We calculated the average systematic errors in four directions: lateral, longitudinal, vertical and rotation. The largest average systematic error was calculated in the lateral direction in the cerebellar area, and the error was also statistically significant (p < 0.05). In longitudinal and vertical direction, we did not find statistically significant differences between brain lobes. In rotational direction we notice the statistically significant difference in frontal lobe (p=0.037) and cerebellar area (p=0.002). The largest radiation margin was calculated for tumors in the cerebellar area (0.80 mm), while for tumors in the parietal lobe the calculated margin was only 0.31 mm. Using linear regression, we found that for lateral movements and rotation, only the position of the tumor in the cerebellar area was statistically significant (p < 0.05), while longitudinal and vertical movements were not affected by any of the factors. Discussion and conclusion: We found that the magnitude of the average systematic errors is less than 1 mm. The reason may be the fixation of patients with three-point fixation masks. Depending on the calculated irradiation boundary (clinical tumor volume and planned target volume), the boundary can be reduced from the existing size of 5 mm to a size between 2 and 3 mm if the portal imaging protocol is followed during irradiation.
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