ESTRO 2021 Abstract Book

S465

ESTRO 2021

Purpose or Objective We aimed to investigate the 3D dose variation caused by the interplay effect during the use of VMAT based lung SBRT without gating beam control. Materials and Methods A programmable BrainLAB ET Gating Phantom was used to simulate breathing motion for different respiratory patterns (sinusoidal, (1-Cosine) 2 , breath hold), different range of amplitudes (50 and 75) and cycle times (3s, 4s and 5s). During the treatment planning, prescription dose was set as 50 Gy in 5 fraction and VMAT based lung SBRT plans were created for two different scenarios. In the first scenario, CT scan of the synchrony phantom was performed for stationary condition and treatment plan, called as "VMAT 1", was created for stationary spherical target (r= 1 cm and volume= 4.18 cc) inside of the 3 ×3 ×3 cm 3 cube phantom as illustrated in Figure 1a. In the second scenario, CT scans were performed for moving phantom and treatment plan, called as "VMAT 2", was created for internal target volume (ITV) (volume= 8.14 cc) defined with the use of created 4DCT dataset for maximum intensity projection (CT MIP ) as shown in Figure 1b. Then, both VMAT plans were delivered for defined conditions without gating beam control. All measurements were performed on Elekta Versa HD linear accelerator using EPID iViewGT panel and iViewDose software. During the evaluation of the dose variation caused by the interplay effect, measured and calculated dose distribution were analyzed using 3D gamma analysis method available in iViewDose software. As an evaluation criterion, 3 mm DTA and 3% DD were used. Pass-fail criteria was based on point dose differences at dose reference point (ΔDRP), the mean γ value, the maximum 1% γ value and the percentage of points with γ ≤ 1 within the 50% isodose surface of the planned maximum dose.

Results As presented in Table 1, the maximum value for gamma passing rate was found in static delivery of VMAT 1 plan as expected (γ ≤ 1: 86.2%). However, the gamma passing rates were outside of the tolerance limit for all other defined conditions. This is due to the fact that the gamma analysis were performed within the 50% isodose surface region of the planned maximum dose calculated inside of the external body. When the target volume was selected as a region of interest during the gamma analysis, the minimum gamma passing rate increased up to 88% and 83% for VMAT 1 and VMAT 2 plans, respectively. Although the dose distribution on the moving phantom showed no significant differences for different cycle times (3s, 4s and 5s). Change in amplitude of respiratory motion cause a dramatic change in gamma passing rate. Additionally, the differences in dose line profile for different motion pattern at beam central axis were illustrated in Figure 2.