Optimum beam angles for the conformal treatment of lung cancer: a CT simulation study

Treatment of lung cancer is often performed with cone-down oblique beams to spare spinal cord and normal structures. However, there is no optimum technique to determine oblique beam angles when a CT simulation is not available. Impact of oblique beam angle was investigated in this study. Fifteen patients with centrally located lung tumors were immobilized and scanned using a CT simulator. The target volumes, left and right lungs, and spinal cord were delineated on each slice. Patients were simulated starting with anterior-posterior treatment beams and subsequently an oblique opposed pair beam from 0 degrees up to 60 degrees at an interval of 5 degrees to optimize the projection of target-to-cord distance and minimize the lung volume in the treatment fields. Analysis was performed with a dose volume histogram (DVH) in each beam orientation. The distance between the target volume and spinal cord was linearly related to the angle of the beam. A larger angle facilitated further sparing of the spinal cord; however, progressively more lung volume was exposed. The 50% DVH data for lung volume was used as an indicator of lung volume. Although, the minimum lung volume was irradiated with an angle of 30 degrees, the additional lung treated increased by only 8 +/- 7% of the total lung volume for 30-60 degrees beam angles and cord distance increased by 18.5 mm. A 30 degrees oblique parallel-opposed beam for the cone-down treatment of lung provided minimum lung volume in the irradiated field; however, the spinal cord distance increased linearly with beam angle. A CT simulator is ideally suited for simulation of lung cancer to maximize the clearance from the spinal cord and minimize the additional lung volume irradiated. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 359-365 (2000).