SU‐E‐T‐658: Quantitative Comparison of VMAT Arc Geometry for Locally Advanced Lung Cancer with Large PTVs

Purpose: Overall low dose to lung for patients with locally advanced lung cancer with large PTVs is a concern, prompting investigation into the preferred arc geometry for lung VMAT. Methods: 8 subjects (6 males, 64±7 years, stage III/IV=5/3) were enrolled with a mean PTV size of 595±303cc. A standard dose scheme of 60Gy in 30 fractions was used for all subjects. For each subject, four VMAT plans with different combinations of the number of arcs (1 arc vs. two arcs) and the arc range (full vs. partial arc) were generated in Eclipse. Dosimetric characteristics for each plan were evaluated by PTV coverage and OAR sparing. PTV coverage was quantified by mean, max dose, homogeneity index HI5/95 defined as (D5%–D95%)/Mean_dose and conformity index (CI). OAR sparing was measured by mean, V10 and V20 for lung minus GTV, mean, max, V30 and V40 for heart and max dose for cord. For each metric abovementioned, a two-way repeated ANOVA analysis was performed to evaluate whether the number of arcs and the arc range will significantly affect dose distribution of VMAT plans. Results: In general partial arc geometry provided better PTV coverage with significantly smaller HI5/95 (p<0.05) and higher mean dose (p=0.15) when keeping the total MU similar. For OAR sparing, partial arc geometry resulted in more dose sparing to the contralateral lung with significantly reduced mean lung dose (p=0.006). On the other hand, full arc geometry significantly reduced mean dose to the ipsilateral lung (p=0.02) as well as dose to the heart and the cord (p<0.05) especially with a two-full arc geometry. Conclusion: Results of this study suggest that choice of an optimal VMAT geometry depends on clinical goals. For example, partial arc geometry should be preferred over full arc when considering overall low lung dose for patients with poor lung functions.