Improving VMAT Delivery Accuracy by Using An Aperture Complexity Penalty During Optimization
K C Younge1*, M M Matuszak1, J M Moran1, D L McShan1, B A Fraass2, D A Roberts1, (1) University of Michigan Hospital, Ann Arbor, MI, (2) Cedars-Sinai Medical Center, Los Angeles, CATH-A-213AB-4 Thursday 8:00:00 AM - 9:55:00 AM Room: 213AB
Purpose: Apertures optimized during VMAT planning can be small and irregular, resulting in dosimetric inaccuracies during delivery. Our purpose is to develop and integrate an aperture-regularization objective function into the optimization process for VMAT, and to quantify the impact of using this objective function on dose delivery accuracy and optimized dose distributions.
Methods: An aperture-based metric ('edge penalty') was developed that penalizes complex aperture shapes based on the ratio of MLC side edge length and aperture area. To assess the utility of the metric, VMAT plans were created for an example paraspinal SBRT case with and without incorporating the edge penalty in the cost function. To investigate the dose accuracy, Gafchromic EBT2 film was used to measure 15 sample individual apertures and composite plans with and without the edge penalty applied. Films were analyzed using a triple-channel uniformity correction and measurements were compared directly to calculations.
Results: Apertures generated with the edge penalty were larger, more regularly shaped and required 22% fewer monitor units than those created without the edge penalty. DVH analysis showed that the changes in doses to organs at risk and normal tissues were minimal. Edge penalty apertures showed a significant decrease in the number of pixels disagreeing with calculation by more than 10%. The number of pixels passing in the composite dose distributions for the edge penalty and non-edge penalty plans were 52% and 96%, respectively. Employing gamma criteria of 3%/1mm resulted in a 79.5% (without penalty) / 95.4% (with penalty) pass rate for the two plans.
Conclusions: The use of the edge penalty during optimization has the potential to significantly improve dose delivery accuracy for VMAT plans while minimally affecting optimized dose distributions.