Purpose: In VMAT for H&N cancer patients, setup and positioning uncertainties of shoulders can impact negatively the dose distribution especially for the lower neck target. In this study, we proposed a novel treatment planning method using a virtual block to improve plan robustness, and evaluated its effectiveness under setup variations.
Methods: One head and neck cancer patient with a lower neck target was used in this study. In Eclipse TPS (v15.6, Varian, USA), a virtual block was created near the humeral head by expanding 15mm from the body contour at the sagittal plane. A plan (vPlan) was created using the block as an avoid structure in plan optimization to prevent x-rays from entering the shoulders and its vicinity. To create realistic shoulder setup and positioning variations, shoulders were deformed with 5mm、10mm、and 15mm in superior-inferior direction and anterior-posterior direction using Thin Plate Spline algorithm in 3Dslicer (v4.11，Brigham and Women’s Hospital, Harvard Medical School, USA) to create the deformed CTs (dCTs). Structures in oCT were deformed to dCTs in Velocity (v4.0, Varian, USA). The vPlan was applied to and recalculated on dCTs to evaluate the variation of dose coverage for the lower neck target and compared to the conventional plan (cPlan).
Results: For both vPlan and cPlan, the variation of dose coverage for the lower neck target were greater in the superior-inferior direction than in anterior-post direction. However, the vPlan shows much less variations compared to cPlan, with the variations of D99，D98，D95 less than 0.6%（0.32%-0.57%）for 15mm SI deformation vs D99，D98，D95 over 2% (2.11%-2.45%) for cPlan.
Conclusion: The proposed treatment planning method using a virtual block is practical to implement and can improve the plan robustness under shoulder setup and positioning variations for H&N cancer treatment using VMAT.
Not Applicable / None Entered.