ESTRO 2020 Abstract Book

S784 ESTRO 2020

Elements). The last six plans were with one isocenter, of which five plans were automatically generated. In v2.0, a jaw tracking of Elekta Versa HD is possible. The maximum allowed gross tumour volume (GTV) to planning target volume (PTV) margin was 1 mm. The prescription dose was 21 Gy or 18 Gy in a single fraction or 8.5 Gy in 3 fractions depending on the PTV volume. All plans were calculated with Monte Carlo, except the VMAT, which was calculated with Collapsed Cone algorithm. The PTV coverage should be at least 98%. To assess SRS plan quality, the Paddick conformity index (CI), the Paddick gradient index (GI), the total V12Gy and V5Gy of uninvolved brain were studied and reported as MEAN±1SD. Additionally, the number of monitor units (MU) and irradiation time were compared. Results The mean total CI was the highest for dynamic IMRT and automated DCA with one isocenter (0.73±0.1) (see table). The lowest GI was for automatically generated DCA plans with one isocenter (v2.0 high and low NT sparing), whereas the highest GI was for VMAT plans (7.0±3.0). The total V12Gy of automatically generated DCA plans with one isocenter (3.3±3.2% and 3.6±3.5% for high and low NT sparing, respectively) were comparable with the manual DCA plans (3.6±3.7%). For the total V5Gy, the automatically generated DCA plans with one isocenter were comparable with the manual DCA plans. The number of MU was the smallest for VMAT plans, followed by automatically generated DCA and IMRT plans and much lower than the manual DCA plans. The irradiation time was the shortest for automatically planned DCA plans. Table: mean results

Conclusion Automatically generated LINAC-based SRS plans for multiple BM with one isocenter can make the number of MU and irradiation time smaller with a comparable or even better plan quality to manual DCA plans with the separate- isocenter approach. Based on all compared parameters, DCA plans with one isocenter were the best and comparable with manual DCA plans with separate isocentres for each metastasis. Moreover, we can conclude that automated DCA plans with v2.0 performed better than v1.6. PO-1466 Dosimetric comparison 3D vs VMAT tecniques in a pediatric population treated for Hodgkin's Lymphoma S. Meregalli 1 , S. Baroni 2 , C. Julita 3 , E. Bonetto 1 , D. Panizza 4 , G. Montanari 4 , F. Colangelo 2 , S. Arcangeli 2 1 Ospedale S. Gerardo, Radiation Oncology Department, Monza, Italy ; 2 Università Milano-Bicocca, Radiation Oncology Department, Monza, Italy ; 3 Ospedale S. Gerardo, Radiation Oncology Department, Monza, Italy ; 4 Ospedale S. Gerardo, Physics Department, Monza, Italy Purpose or Objective To retrospectively compare the dose distribution to non- targeted tissues in pediatric patients treated for mediastinal Hodgkin's lymphoma (HL) using 3D conformal radiotherapy (3D-CRT) and Volumetric Arc Therapy (VMAT).