(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Estimate treatment doses using daily CBCTs, to evaluate the robustness of VMAT plans to residual alignment inaccuracies that cannot be corrected with rigid shifts for simultaneous multi-target (breast/lumpectomy and IMNs) alignments.
Methods: A workflow was developed (in MIM) to create synthetic daily CT images (dCT) from the CBCTs for dose calculation. The planned dose was recomputed on dCT in the aligned position and then cast back on the planning CT for evaluation; this eliminated the need for daily contouring. The conventional alignment strategy is to split the differences between aligning the breast/lumpectomy and the IMNs; the method is prone to user variability and inconsistency due to the subjective nature of the trade-off. As such, several “single-target” alignment strategies have also been investigated for their surmised consistency over time and among users: breast-only, IMN-only, rib-arch alignments near the ipsilateral breast. The planning goal for targets was D95>95%, with D90>90% considered an acceptable variation.
Results: The quality/accuracy of the dailyCT images were deemed appropriate by visual inspection. All alignment strategies performed similarly, when averaged over all fractions. The dosimetric coverage of the structures of interest (targets, OARs) was degraded due to imperfect alignments under all scenarios. D90 values were always met, possibly because the treatment plans exceeded the planning directives. This was not always the case with D95. Doses to OAR were less sensitive to alignment errors.
Conclusion: Averaged over the entire treatment course, various strategies for aligning multiple targets yield similar cumulative doses. Doses to the relevant organs at risk are not sensitive to the daily anatomical variations. Due to high gradients surrounding the targets, their dosimetric coverage is more sensitive to changes in anatomy and setup inaccuracies, and may warrant treatment adaptation. This patient dose delivery QA methodology will help identify challenging patient setups that may require treatment adaptation.
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