ESTRO 2024 - Abstract Book

S3705

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

2752

Digital Poster

A phantom study of robust analysis of Lung SBRT BH plan

George Zhao, Xin Wang, Julianne Pollard-Larkin

MD Anderson Cancer Center, Radiation Physics, Houston, USA

Purpose/Objective:

a phantom study to analyze the robustness of breath hold Lung SBRT treatment. When treating a moving lesion, breath hold is sometimes necessary to stabilize the lesion so that radiation can be delivered to the lesion according to the plan. Due to patient condition, BH is often not as reproducible as desired, a residual tumor motion is often observed. The present study analyzes the robustness of a BH Lung SBRT plan with a residual tumor motion of 5 mm in superior and inferior direction and we propose an CBCT alignment strategy to minimize the dosimetry impact to tumor due to the residual tumor motion.

Material/Methods:

A CRIS lung phantom with a 1 cm diameter tumor insert was scanned and two lung SBRT plans, plan A and plan B, were generated. In plan A, the enhancing tumor was contoured, and its contour was expanded superiorly (or alternatively in inferior direction) by 5 mm to make the GTV. In plan B, the tumor contour was expanded superiorly and inferiorly by 2.5 mm to make the GTV. The PTV was created based on the GTV with 5 mm uniform margin. The GTV is planned to 60 Gy in 4 fractions and PTV to 50 Gy using VMAT technique. Plan A and B both assume the same 5 mm the residual tumor motion but different tumor location in the GTV contour. See Fig. 1.

Fig.1. Left: plan A; Right plan B. Red: Tumor; Maroon: GTV; Cyan: PTV

A robust evaluation was performed in treatment planning system in plan A and B with 5 mm isocenter shifts in either R-L, I-S or P-A directions, one direction at a time, 7 scenarios for each plan A and B including the plan with zero shift. The tumor coverage of these plans was evaluated.

Results:

Table 1 shows the coverage of tumor, V95% at 60 Gy. It was 99.51% and 100.00% for plan A and B respectively without iso shifts. with 5 mm iso shifts, the range of V95% was 85.2% to 100% (mean = 93.5%, Std Dev = 5.13) for