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Dr. Lütfi Kırdar Kartal Eğitim ve Araştırma Hastanesi, Radyasyon Onkolojisi Bölümü, İstanbul It is difficult to obtain a homogenous dose distribution with fixed electron field in patients who have uneven chest wall surface following mastectomy due to the factors such as tissue thickness differences within the irradiation field, irregular contour, long insicion scar and field matching problems. Electron arc therapy is proposed as an alternative technique in such patients. For this purpose; relationship between electron energy and arc angle and x-ray contamination during electron arc therapy was investigated. In this study Saturne-41 linear accelerator with 6 to 16 MeV electrons, X-OMAT-V verification films, cylindrical phantom and pixel manual densitometer were used. Verification films which were irradiated in a cylindric phantom with an isocentric technique by using 5x30 cm2 field size, 0°, 60°, 90°, 120°, 180° arc angles and 6, 9, 13.5 and 16 MeV electron energies were evaluated with a densitometer. Isocentre depth was selected at 15 cm. Depth dose percentage curves obtained from densitometer revealed a trend towards increased dose around isocentre. This difference was up to 26.5% above the maximal dose depending on the energy and arc angle. Increase of the dose was 22.5% at 180° arc angles and 5.5% at 0° angle for 6 MeV electron energy. Similarly when 16 MeV electron energy was used, increase of the dose was 10% at 0° and 26.5% at 180° arc angles. Increase of the dose is due to bremss rays contamination (x-ray). Electron arc therapy plays an important role in the treatment of superficial tumours on curved surfaces. In this technique as the dose increases around isocentre, critical structures should be considered when selecting an isocentre point and x-ray contamination should be taken into account during treatment planning. Keywords : electron arc therapy, x-ray contamination