ESTRO 2021 Abstract Book

S1601

ESTRO 2021

As regards of target robustness, the mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and

3.7% (PTV). Conclusion

The dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size. This approach can prevent the exclusion of patients with large cranio-caudal therapy volumes from the treatment. PO-1881 Automatic VMAT treatment planning for left-sided breast cancer with lymph nodal involvement. K. Engstrøm 1 , C. Brink 1,2 , M.H. Nielsen 3 , M. Kjellgren 1 , K.L. Gottlieb 4 , I. Hazell 5 , V.N. Hansen 1 , E.L. Lorenzen 4 1 Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark; 2 Department of Clinical Research, University of Southern Denmark, Odense, Denmark; 3 Department of Oncology, Odense University Hospital, Odense, Denmark; 4 Laboratory of Radiation Physics , Department of Oncology, Odense University Hospital, Odense, Denmark; 5 Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Denmark, Odense, Denmark Purpose or Objective Conventional radiotherapy for breast cancer is tangential forward planed fields with segments. This is often a manual and time-consuming process, which may be more consistent with automatic planning. Treatment techniques such as Volumetric Modulated Arc Therapy (VMAT) can be used to increase the coverage and conformity, but may increase the dose to contralateral organs. In the present study, we evaluated VMAT in a butterfly field configuration using Pinnacle Autoplan module in comparison with forward planned tangential plans (3DCRT) in terms of dose to organs at risk, target coverage, the number of breath holds required for treatment in deep inspiration breath hold and dosimetric accuracy. Materials and Methods Twenty left-sided breast cancer patients, ten with mastectomy and ten with lumpectomy, were used for the study. Seventeen of the patients were treated in deep inspiration breath-hold. Patients were prescribed 50 Gy to the CTV’s. For all patients, a VMAT and a forward tangential plan were generated. The forward plan used step-and-shoot 6 MV combined with 18 MV fields for larger breasts. The VMAT plan was made using two 6 MV VMAT fields in a butterfly configuration (each arc spanning 40-70 degrees). A skin flash of 1cm was used to ensure robustness for the VMAT plans. Dose planning was done in Pinnacle 16.0 using the Auto-Planning module. Comparison of 3DCRT and VMAT was made by evaluating the difference in dose to OAR and targets in terms of mean dose, metrics provided by DBCG guidelines 1 , dose-volume histograms, and required breath-holds for treatment delivery. Results Similar dose to OAR was achieved with VMAT compared to 3DCRT (Figure 1). Small differences were observed in the mean dose to the heart and humeral head. The heart dose was slightly reduced with VMAT, whereas the dose to the humeral head was increased (Table 1). Target coverage was similar, however, with a small statistically significant increase in coverage of supraclavicular- and internal mammary nodes CTV’s (V 45Gy ) for VMAT. Compared to 3DCRT, VMAT reduced the number of needed breath-holds during treatment by 56 %. All the VMAT plans were measured with ArcCHECK and clinically acceptable. Measurements had a mean gamma pass rate (3 mm and 3 %) of 98.4 % with a range from 96.6 % to 99.8 %. Conclusion Auto-planning of VMAT within the treatment planning system Pinnacle produced plans in a fast and consistent manner requiring less interaction from the treatment planner, thus potentially reducing inter-planer variations. The dose distribution of the butterfly VMAT technique was similar to that for the conventional tangential irradiation. The VMAT