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Uludağ Üniversitesi, Radyasyon Onkolojisi ABD, Bursa Medulla spinalis (MS) is the critical restrictive organ in mediastinum irradiation. Oblique fields are usually designed excluding the cord in boost irradiation. Purpose of this study is to obtain optimum oblique angles as defined by coverage of the target volume and to assess its advantages or disadvantages by choosing different oblique angles. Seventeen patients with lung cancer were evaluated postoperatively. All patients were irradiated with AP/PA fields to a dose of 45 Gy and then oblique boost fields were planned isocentrically. Median dose was 54 (50.4-59.4) Gy. The contralateral mainstem bronchus (CMB) and subcarinal space (SS) were defined as the target volume and new oblique boost angles were designed. The median SA measured was 30° (range 21 °-36°) for the anterior oblique fields and 23° (10°-34°) for the posterior oblique fields in old plans. In new plans, the median SA was 48° (43°-53°) and 42° (25°-50°), respectively with statistical difference (p<0.001). CMB and SS had better dose homogeneity in new plans (p=0.004, p=0.002). Irradiated opposite lung volume increased with statistical significance in new plans (p=0.03), but median maximum dose was not found significant (p>0.05). The increase in median MS dose was close to statistical significance in new plans (p=0.049). To adequately encompass the entire mediastinum as defined in the Intergroup Trial (RTOG 91-05) with offcord oblique fields treatment angles greater than 40 degrees are necessary. Our results demonstrated that oblique field angles must be greater than 40 degrees. The potential increase in lung volume exposed to radiation should be taken into account with these angles. Keywords : mediastinum irradiation, radiotherapy, medulla spinalis, gantry angles