Novel Treatment Planning Techniques for Treatment of Chest Wall Lesions using Stereotactic Body Radiotherapy

Abstract

Materials/Methods: Ten patients with the target volumes attached or near to the chest wall were chosen for this study. Treatment plans using three different techniques were performed retrospectively. These three techniques included conventional 3D planning (3D), forward planning with manual optimization (Field in Field (FIF)) and segmented beams with inverse planning optimization (SIP). A 4DCT simulation was performed using a Philips 16 slice CT scanner. All patients were simulated in the supine position with arms placed above their head and immobilized using a body frame or body fix. These 4D-CT images were fused with helical CT scans and were primarily used for target delineation. Note that all the treatment plans were performed on the free breathing helical CT scans. Plans involved anywhere between 7 -11 noncoplanar beams. 3D conformal plans were designed by manually creating all beam apertures using a uniform margin around the PTV and adjusting the block shape for avoiding critical structures. For FIF technique, 2-3 segments were created for each field. A segment was created by manually moving the MLC leaves into the aperture from the original block edge effectively blocking out an area where too much dose was being delivered. SIP technique uses the same beam angles as the two other techniques. Plans were optimized using direct machine parameter optimization, allowing 1-2 segments per beam. A minimum of 50 to 100 monitor units along with a minimum segment size of 5 to 10 cm was used. Additional structures and optimization constraints were used for inverse planning. The following criteria were used for plan evaluation. All treatment plans had to adhere to the RTOG 0236 protocol. Volume of the PTV covering 90% prescription dose should be at least 99%. Conformality index (CI) was defined as the ratio of the volume of prescription isodose to the volume of PTV. Plans were evaluated for plan quality and time efficiency.