Purpose: To determine the impact of rotational uncertainties on dosimetric coverage of distant targets using VMAT planning and a TPS tool to measure plan uncertainty
Methods: VMAT plans containing a spherical 8 mm GTV with uniform 1 mm PTV expansion at 5 cm from isocenter were generated in a phantom. Similar plans were also generated for a target at 6 cm to assess differential impact based on distance. All plans were normalized such that V100%> 98% for each PTV; CI ranged from 1.23-1.32. As the TPS tool for assessing plan uncertainty uses translational shifts only, shifts were calculated to represent the location of the target after rotations of 0.4 and 0.5 degrees. PTV and GTV dosimetric coverage was then analyzed to determine the impact of each rotation in pitch, roll, and yaw individually. The impact of positive versus negative roll was also evaluated to determine sensitivity to rotational direction at each distance.
Results: GTV coverage remained at 100% for all tests. In no case did the d=5 target coverage fall below 93%, whereas in 2/6 cases the d=6 target did. Worst-case coverage for the d=5 target was 93.0% versus 88.3% for the d=6 target. Dosimetric impact was found to be sensitive to rotational direction for the d=6 target, with coverage suffering more with roll in one direction versus the other.
Conclusion: Rotational deviation reduced PTV coverage at both distances evaluated. The effect was larger for greater degrees of rotation and was more variable at greater distance. Despite decreased PTV coverage, the 1 mm GTV to PTV margin ensured that GTV coverage remained at 100%. This data demonstrates that target coverage may be sensitive to uncorrected rotation. The potential impact of rotational deviations should be considered when evaluating targets far from isocenter with a specific GTV to PTV margin.