ESTRO 2021 Abstract Book

S1586

ESTRO 2021

challenging case of online adaptive MRgRT. Finally, in phase 3, it has been evaluated the evaluation of LAPC VML robustness and comparison with clinical IMRT plans, when a lateral positioning error of ± 5 mm is manually introduced. The comparison between IMRT and VML plans was performed considering the following parameters: number of beams, number of IMRT segments beam on time, total treatment time, target coverage (PTV V95%>95%), hot spots (PTV V105<5%) and OARs constraints. In addition, to evaluate the low dose spread, a region of interest named “low doses” (LD) has been generated for each plan, as the difference between patient’s body and the Ring structure around the PTV of 3 cm thick. Finally, IMRT and VML plans were delivered on a 3D detector and the measured dose distribution was compared to the calculated one by the treatment planning system (TPS) in terms of global gamma analysis. Results In phase one, all VML plans were within constraint for all OARs. When PTV coverage is considered, in the 50% of the cases VML PTV coverage is equal or higher than in IMRT plan. In the remaining 50%, the highest target under coverage difference in comparison with IMRT plan is -1.71%. The mean and maximum treatment time differences (VML-IMRT) is 0.2 min and 3.1 min respectively. The VML gamma analysis resulted always within the tolerance and in 60% of the cases showed higher gamma passing rates when compared to the IMRT plans. Results of phase two are reported in figure 1 and 2.

As regards of phase 3, no significant differences have been observed between the VML and IMRT shifted plans. Conclusion VMAT-like treatment approach appeared to be an efficient planning solution and it was decided to clinically implement it in daily practice, especially in the frame of hypo fractionated treatments. PO-1862 Revisiting whole brain radiotherapy standards: Is it time to abandon opposed lateral fields? E. Oymak 1 , M. Polat 2 1 Iskenderun Gelisim Hospital, Radiation Oncology, Hatay, Turkey; 2 Iskenderun Gelisim Hospital, Radation Oncology, Hatay, Turkey Purpose or Objective To evaluate doses for organs at risk (OARs) in whole brain radiotherapy (WBRT) in the standard opposed lateral fields (OLF) compared with MLC with IMRT and VMAT plans. Materials and Methods A single-institution retrospective analysis was conducted. Planning CT images of patients treated with WBRT were obtained