SU-E-J-120: A Phantom-Based Comparison of Lung Tumor Planning Target Volumes and Organs a Risk Dose Reduction Between 4D-CT and 3D-CT

Abstract

Purpose: Lung tumor motion due to respiration necessitates using large margins when creating a planning target volume (PTV). As tumor movement of up to 5 cm has been reported by several investigators, accurate investigation of the internal margin (IM) to 3D CTV is necessary. The purpose of this study was to compare the differences in volume between PTVs generated using 4D‐CT (4D PTV) and 3D‐CT (3D PTV) for varying degrees of tumor motion and to evaluate the differences in dose to normal lung and spinal cord. Methods: A movable phantom was used to simulate lung movement (Figure 1). Three blocks of rubber were attached to the phantom and a 3D‐CT and a 4D‐CT were taken. Then, PTVs were delineated and compared. After that, the PTVs were transferred to a Rando phantom for dose analysis. Also, investigation of varying displacements of lung tumor was done using four blocks of rubber attached to a movable phantom with different known displacements (Figure 2). Results: The average reduction in PTV volumes using 4D‐CT was 33.8%. This led to average decreases of 19.2%, 20.1% and 33.8% for lung V20, mean lung dose and maximum spinal cord dose, respectively (Table 1)., Investigation of varying displacements of lung tumor shows that if tumors move more than 4 cm, the 4D PTV may be equal to or larger than the 3D PTV(Table 2).Figures 3 shows overlapping of PTVs of tumors with 2 cm motion and 4 cm motion in 3D and 4D. Conclusion: It can be concluded that 4D‐CT reduces organ at risk doses. For lung tumors with large displacements, 4D‐CT is a promising method because the exact IM in the direction of tumor motion is achieved. In these cases, the use of 3D causes not only unnecessary dose to normal tissue but also missing part of the tumor (Figure 3).