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SU-E-T-286: Robustness of One- Versus Two-Isocenter SBRT for Two Lung Lesions with Rotational Setup Errors.

PURPOSE
Evaluate the robustness of two different planning techniques for SBRT patients with two lung lesions: with both PTVs treated simultaneously with one isocenter, or separately with an isocenter for each PTV.


METHODS
One- and two-isocenter IMRT and VMAT SBRT plans were created in Varian Eclipse for five patients who each had two lung lesions. PTV coverage for the one- and two-isocenter plans was similar when possible. Rotational setup errors were simulated using couch rotations of various angles, and the resulting dose distributions were calculated.


RESULTS
With a 2° setup error, the average decrease in PTV volume receiving at least the prescribed dose (V100%) was 1.66% for the oneisocenter plans, and 0.05% for the two-isocenter plans. The changes in V100% ranged from +0.33% to 4.06% for one-isocenter plans, and from +0.23% to -0.30% for two-isocenter plans. With a 5° setup error, the average decrease in V100% was 6.46% for a one-isocenter plan (range: -0.19% to -13.18%), and 0.42% for a two-isocenter plan (range: +0.21% to -1.33%). For one-isocenter plans, the decrease in V100% tended to increase as PTV separation increased. The three patients with PTV separation less than 10 cm each showed a decrease in V100% of less than 1% for a 2° error, while the two patients with larger PTV separations showed decreases of more than 3%.


CONCLUSION
As could be predicted by geometric calculations, when a rotational setup error occurs, the PTV coverage decreased more for a one-isocenter plan than for a two-isocenter plan. These findings suggest that, in general, for lung SBRT patients with two lesions, the two-isocenter technique may be preferable. However, they may also suggest that if the PTVs are separated by less than 10 cm, and there is high confidence that setup errors are small (≤ 2°), a one-isocenter plan may deliver similar coverage.