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Trakya Üniversitesi, Tıp Fakültesi, Radyasyon Onkolojisi AD, Edirne This study aimed to determine the variability in treatment set-up during a 5-week course of tangential breast treatment for patients immobilized with a breast board, to compare with the percentage of lung volume irradiated and the central lung distance measured on the simulator and port films. The ipsilateral lung volumes included within the target volume were calculated by a mathematical formula with the base of central lung distances measured at 176 simulator and port films taken from 44 patients. The mean distances on simulator and port films for all patients were 1.52 cm (0.9-2.3) and 1.56 cm (1-2.5) respectively and the mean percentage of ipsilateral lung volume included within the target volume was 6%. The variations between central lung distances measured on simulator and port films were under 4 mm for all patients. For right and left percentage of lung volumes irradiated were 6.64% and 6.33% and there was no difference. In conclusion, using a correct immobilization system the set-up variations may be reduced to minimum and it was confirmed that central lung distance is a safe parameter for determining indirectly the ipsilateral lung volume irradiated. Keywords : breast cancer, radiotherapy treatment planning, lung volume, set-up error