ESTRO 2020 Abstract book

S1093 ESTRO 2020

conformal techniques like Intensity Modulated Radiotherapy Therapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) this can lead to under-dosing of the target volume. Common workarounds, like virtual bolus or “skin flash” methods, often are not robust enough to prevent this. However, the dosimetric consequences of swelling of the breast might be mitigated by using robust optimization. In this study we investigated if there is a potential gain for robust optimization for elective breast radiotherapy. Material and Methods Retrospectively, eleven patients with swelling of the breast with more than 0.8cm, as measured on the CBCT, were selected. The prescribed dose for the breast CTV was 40.05Gy in 15 fractions. A PTV margin of 5 mm was used. The clinically used plan was used for this study, which was an IMRT plan with two tangential fields (6MV and/or 10MV). For each patient a non-robust plan, according to the clinical workflow with a skin flash method was made. For creation of a robust plan, a new feature called simulated organ motion, was used. Four synthetic CT-scans were generated by deforming the breast CTV in different directions towards the air over a distance of 1cm on average. These synthetic CTs were used as scenarios during optimization, besides the original planning CT. Evaluation of the planned dose was done on a repeat CT- scan, when available, or else on a synthetic CT-scan generated by deforming the CBCT to the planning CT. Both CT-scans were registered to the planning CT-scan with only translations and no rotations according how the patient would be treated. The difference between planned dose and evaluated dose of the coverage of the breast CTV V 95% (of the prescribed dose) and the D 1cc (Gy) (dMax) was evaluated. The difference of the V 95% between the planned and the evaluation dose, of both non-robust and robust plans, was tested with a Wilcoxon signed-rank test with a p-value of 0.05. Results For eight out of eleven patients V 95% coverage with the robust plan was better than with the non-robust plan (P=0.026). The average difference between planned and evaluated dose of the V 95% was -0.88% and 0.02% for the non-robust and robust plan, respectively. The differences for each patient are shown in figure 1. For five out of eleven patients with a non-robust plan the V 95% coverage decreased with more than -1% and only one out of eleven patients with a robust plan. In figure 2 an example is shown for which the coverage of a non-robust plan isn’t enough but for a robust plan it was. The D 1cc increased with an average of 0.6Gy and 0.8Gy, for the non-robust and robust plan, respectively.

Conclusion Organ motion-based robust optimization results in more robustness against swelling of the breast than using a skin flash method. PO‐1865 Quality control of SBRT treatments with VMAT J.A. Válgoma Solanas 1 , A. Montejo Arteche 1 , M. Fernandez Montes 1 , N. Pereda Barroeta 1 , N. Sánchez Hernández 1 , M. Castanedo Álvarez 1 , F.J. Conejo Monasterio 1 1 Hospital Universitario Basurto, Radiotherapy, Bilbao, Spain Purpose or Objective As radiotherapy techniques have been increasing in complexity, prescription dosesare higher and dose gradientsare steeper, the quality controls (QC) of the treatments have become more exhaustive in order to guarantee their correct administration. In particular, the QC are something fundamental in the radiotherapy process of SBRT due to the high doses per fraction administered in this type of treatments. In this work the controls carried out for this type of treatment are exposed and analyzed. Material and Methods Two types of QC are distinguished, on the one hand those that analyze the physical and dosimetric parameters of the LINAC (Machine QA) and, on the other hand, those that are carried out to validate the treatment plan of each patient (Patient QA).To carry out the Machine QA weekly, image controls (CBCT, MV and KV), field size, MLC (Picket Fence), a star test, dose rate control, gantry speed and MLC speed are performed. In addition,Winston-Lutz test is also carried out every day a SBRT is programmed, which allows us to check the accuracy of the isocenter before treatment. All these measures have been carried out with the SNC Machine (Sun Nuclear) web platform.Regarding the Patient QA, for each treatment plan we perform three measurements: one-point dose, with a cubic phantom of