Purpose: To investigate planning strategies for head-and-neck radiation using volumetric-modulated arc therapy with robustness against interfractional shoulder motion and to evaluate each approach for reduction of shoulder dose and planning efficiency.
Methods: Ten patients were randomly selected for planning in RayStation v10A. Clinical plans served as base plans and had no mechanism for robustness against interfractional shoulder motion. Plan variations included the following: ‘shoulder-protect’: using a feature in the TPS that closes the MLC leaves over a selected structure to avoid entrance dose, and ‘shoulder-PRV’: adding shoulder-avoid structures to the optimizer. Two patients were also planned using ‘skip-arcs’: partial arcs which skip the segments around the shoulders, ‘shifted-iso’: shifting the isocenter and utilizing beam divergence to avoid beams passing through the shoulders, and ‘couch-kicks’: using couch rotations to avoid the shoulders. Plans were evaluated for target coverage, shoulder sparing, OAR doses and planning efficiency.
Results: V95 target coverage for all variations was within 2% of the base plan. All variations achieved lower mean shoulder dose relative to the base plan: 73%(shoulder-protect), 71%(shoulder-PRV), 80%(skip-arcs), 94%(shifted-iso), 59%(couch-kicks); however, the dose to 1% of the shoulder structures (D1 shoulder) increased relative to the base plan: 113%(shoulder-protect), 107%(shoulder-PRV), 109%(skip-arcs), 103%(shifted-iso), 104%(couch-kicks). The complexity metric (ratio of MU’s over prescription dose) remained similar, except for an increase for the ‘shoulder-PRV’ approach(109%) and a reduction with the ‘couch-kick’ approach(88%). Dose homogeneity increased for the ‘shoulder-protect’(108%) and ‘shoulder-PRV’(106%) variations and decreased for the ‘skip-arcs’(90%), ‘shifted-iso’(94%), and ‘couch-kick’(99%) variations. The ‘shoulder-PRV’ variation required the least amount of manual adjustments and planning time.
Conclusion: Adding shoulder-avoid structures directly to the optimizer was found to be the best compromise between increased plan robustness against interfractional shoulder motion and increased planning complexity. More cases will be studied in the future to confirm the statistical significance of the findings.