ESTRO 2023 - Abstract Book

S823

Monday 15 May 2023

ESTRO 2023

doses were 38.5 Gy/10 fr for APBI and 27 Gy/3 fr for SBRT for spinal lesions. The primary endpoint was the protocol treatment completion rate, whose definition was the completion of the scheduled number of DWA with the patient-specific quality assurance (PSQA). The PSQA of DWA was performed using ArcCheck (Sun Nuclear Corp.), and the gamma analysis criterion of 5%/1 mm and 3%/3 mm was used for all plans. The secondary endpoint was the acute adverse event rate during the first 90 days after the start of irradiation. In APBI, non-coplanar three-dimensional conformal radiotherapy (3D-CRT) and coplanar VMAT treatment plans were created for comparison with the DWA plans, which were equal to or superior to that of the 3D-CRT and VMAT. For spinal lesions, treatment was performed with DWA when it was equal or superior to coplanar VMAT.

Results Twenty-five patients were enrolled in this trial, including 12 with left breast cancer and 13 with spinal metastasis. The median age was 67 (range, 42–85) years. In all 25 cases, no DWA plans were inferior to the 3D-CRT and VMAT plans in quality. Protocol treatment was completed by DWA, and the treatment plans were clinically irradiated without collision in all fractions. The mean of the average passing rates was over 95% for DWA, which met the PSQA criteria. There were no Grade 2 or higher acute adverse events. Conclusion The modified track Dynamic WaveArc therapy was feasible for APBI and spinal lesions, which allowed safe treatment in all patients with minimal acute adverse events. Purpose or Objective Treatment planning in heterogeneous media has always been challenging. Although the accuracy of dose calculation algorithms has significantly improved, dose deposition in near-air density areas remain problematic. In some patients, the lung density is close to air properties (0.01-0.05g/cm3). In absence of tissue mass, the treatment planning system is forced to locally deliver much higher output in order to provide sufficient target coverage. The purpose of this study is to evaluate and validate the use of density override (DO) during volumetric modulated arc therapy (VMAT) planning for lung stereotactic body radiotherapy (SBRT). Materials and Methods 10 SBRT patients with low lung density are included in this study (table 1). For all patients, an internal target volume (ITV) was delineated, using all breathing phases. The margin area between ITV and PTV (ITV-PTV, 5mm) was overridden with a lung density of 0.15 (~-850HU) and 0.26g/cm3 (~-760HU, default mass density value for lung in RaySearch, RS), and 2 new plans were generated using the same constraints, respectively. Each plan was optimized on the average CT in RS such that 100% of the prescribed dose covers 95% of the PTV. Several DVH parameters and the monitor unit (MU) efficiency were compared between the 3 plans. Later, the DO is undone and robust evaluation was performed (standard functionality in RS) in 8 different scenarios with combination of 2mm shifts in all 3 translational directions. A plan is considered robust (pass) if 98% and 100% of the ITV were covered by the 95% and 90% of the prescription dose, respectively. The Dmax of the PTV and OAR of the worst case scenario were also compared. PD-0975 Evaluation and validation of density override in VMAT treatment planning for lung SBRT C.L. Ong 1 , E. Franken 1 1 HagaZiekenhuis, Radiotherapy, Den Haag, The Netherlands