(Saturday, 4/4/2020) 10:30 AM - 12:30 PM [Mountain Time (GMT-6)]
Purpose: For comprehensive breast cancer radiotherapy, VMAT has demonstrated dosimetric advantages over 3D techniques, but robustness of these plans remains in question. We sought to evaluate MCO-VMAT robustness features in Raystation v8a as compared to clinical plans and non-robustly optimized MCO plans in light of clinical residual mismatches and setup errors in comprehensive breast cancer radiotherapy.
Methods: We generated 3 base plans - Clinical, MCO and Robust-MCO for 11 randomly selected patients. Robust-MCO plans were created assuming 5mm deformations of the PTV in the S-I and L-R directions. The base plans were matched at the CTV-D99(%) level and compared using a Wilcoxon paired rank sum test on target coverage, OAR sparing, and plan generation time. The three treatment plans were then applied to planning CT data sets deformed to match the worst CBCT obtained during treatment. Each plan was recalculated on this deformed data set and target coverage was evaluated.
Results: In base plans, the average CTV-D99(%) was 97.1% of the prescription dose [94.3-99.7%]. Considering PTV-D95(%), Robust-MCO (average 97.6% Rx) and clinical plans (average 97.8% Rx) had significantly better coverage as compared to the MCO plans (average 96.8% Rx), but with minimal absolute difference. OARs sparing was significantly worse in Robust-MCO for ipsilateral lung V20 and V10, heart V25 and mean dose (all p<0.04). Robust-MCO plans took, on average, 3 times as long as MCO plans to generate. When evaluated on deformed data sets, Robust-MCO plans retained significantly more coverage of the CTVs at the D99(%) and D98(%) levels as compared to the clinical (p<0.01 and p<0.02) and MCO (p<0.01 and p<0.02) plans.
Conclusion: Robust-MCO plans can be generated for comprehensive breast cancer patients and offer improved target coverage in the presence of residual mismatches and setup errors, though the cost in plan generation time and OAR sparing is significant.
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