SU‐E‐J‐121: A New 4D Radiotherapy Planning Strategy Using Synthesized Tumor‐Motion‐Compensated Computed Tomography

Purpose: To validate a four‐dimensional radiotherapy (4DRT) planning based on a synthesized CTimage compensating for tumor motion, accounting for tumor rotation, deformation and distortion due to motion artifacts, and producing realistic normal tissue density in motion‐tracking beam eye view. Methods: 4D computed tomography (4DCT) images of six patients with peripheral lung lesions in mid or lower lobs (motion range: 0.5–3.5cm and size: 1.5±2.0 cm3) were used. A customized program was used for simulating the patient anatomy with a motion‐compensated tumor using 4DCT by aligning the tumor and averaging the 4DCT into a static 3.5DCT image. The gross tumor volume (GTV) was delineated semiautomatically using a threshold algorithm. Variation of GTV in each phase CT was assessed across all phases. 3DRT plans were generated using 3.5DCT and 4DCT and compared for validation. An integrated dose volume histogram (iDVH) from all phase plans and dose warping using deformable image registration (DIR) were used for evaluating 4D plans and comparing with 3.5D plans. Results: The range of tumor volume variation over the mean within a breathing cycle was 87%±46%. The 3.5DCT produced an ‘averaged’ GTV, more reliable than that from any phase CT. The results show that the 3.5D plan is equivalent to the 4D plan, except for low dose area, using the iDVH evaluation. On average, the percentage difference for the areas under the DVH and iDVH is 4.3%±2.7%, while 2/3 of the difference results from low dose region blow D20%. Using DIR‐based dose warping, PTV coverage varies due to DIR uncertainty for the small lesions. Conclusions: The 3.5D plan is equivalent to the 4D plan for peripheral lung lesions, yet requires much less clinical workload. The 3.5D plan accounts for tumor motion and tumor variation for a more reliable delineation, and for realistic normal tissue representation for motion tracking.