A Magnelli*, B Guo, P Qi, Y Cho, T Gray, S Chao, J Suh, P Xia, Cleveland Clinic Foundation, Cleveland, OH

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  A Magnelli

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PO-GePV-T-398 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Stereotactic body radiotherapy (SBRT) to the cervical and upper thoracic spine is prone to setup uncertainty and patient motion due to the curvature of the neck in this area. Intrafraction motion as high as 1.5 mm has been observed during treatment. The objectives of this study were to evaluate the robustness of our treatment plans against this magnitude of motion and to determine its dosimetric effect on plan quality.

Methods: Ten clinically delivered treatment plans were selected and loaded into the RayStation treatment planning system. All patients were treated for spinal metastases to vertebral levels T5 and above and were immobilized using a 5-point thermoplastic mask. The plans were evaluated for plan robustness against clinical criteria for tumor coverage (V18Gy > 90%) and spinal cord dose (D0.03cc < 14Gy, V10Gy < 10%) using an isotropic misalignment of 0.15 cm. For each plan, the misalignment scenario with the worst dosimetric effect on plan quality was noted and the resulting plan metrics were tabulated.

Results: Mean tumor coverage loss was 3.1% ± 1.4%; plan criteria were violated in 50% of misalignment scenarios. Mean increase in spinal cord D0.03cc was 284 ± 46 cGy; 70% of misalignment scenarios violated plan criteria. Mean increase in spinal cord V10Gy was 10.0% ± 3.7%; 75% of misalignment scenarios violated plan criteria.

Conclusion: The data show that patient motion or setup error of 1.5 mm can cause a plan to exceed dosimetric tolerance in a majority of cases. Further steps toward improvement are warranted including monitoring intrafraction motion, improving immobilization, incorporating robustness in planning, or performing online adaptive planning.

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