ESTRO 2021 Abstract Book

S20

ESTRO 2021

between margin and robust plans were smaller in the 1mm plans. For the ipsilateral parotid, the robust plan was more successful at sparing it than the margin plan for most patients for the 3 and 5mm plans. In general, the largest difference between a margin plan and the corresponding robust plan is seen in the contralateral parotid.

Conclusion We found that 5mm plans created on a commercial TPS achieved target coverage in ~75% of cases, i.e., adaptive radiotherapy would be required in 25% of cases if set-up positioning is perfect. Although they are considered smaller than set-up errors, anatomical changes are important. During this study, we came across a shape effect in the robust plans, where they performed worse for complex shapes than simple ones. We intend to investigate further whether the lower performance of robust plans compared to margin plans for 3 and 5mm may be related to target shape and sampling of error only across the cardinal axes.