ESTRO 2020 Abstract Book

S1042 ESTRO 2020

Radiotherapy (SBRT) for the Treatment of Multiple Metastases. 2015: ClinicalTrials.gov NCT02206334. PO-1869 Dosimetry impact of manual vs. automated segmentation of lung contours in RayStation (RS) F. Vallejo Castañeda 1 , M. Hinse 1 1 Centre intégré de cancérologie de Laval, Département de radio-oncologie, Laval, Canada Purpose or Objective To investigate the dosimetric impact of manual vs. automated segmentation of lung contours created by ATLAS base segmentation (ABS) and model base segmentation (MBS) considering specific dose constraints and contouring creation time. Material and Methods A total number of 45 lung patients was used for the study. 20 of them were used to create an ATLAS while the remaining 25 were used for validation. The original RS MBS was used where no patient data was required. Each validation patient had both lungs contoured manually in Pinnacle and further exported to RS. In RS two other sets of lungs contours were created using ABS and MBS. Lungs- GTV and lungs-PTV contours were generated for dosimetric comparison using the following minor to major toxicity in house dose constraints: 35%

Results It was possible to create pre-treatment plans that met all dose constraints in 10/12, 9/12 and 6/12 patient cases for the 5x7Gy, 3x10Gy and 1x20Gy scheme, respectively. Violations were caused by OARs overlapping or adjacent to the PTV. Dose criteria were violated in the online plans of 5x7Gy 0%(=0/30), 3x10Gy 0%(=0/30) and 1x20Gy 43%(=13/30). The resulting 5x7Gy and 3x10Gy treatment plans were comparable (Figure 1). The 1x20Gy treatment plans resulted into a significantly lower HI, but significantly higher R 100% , R 50% and D 2cm (Wilcoxon matched- pairs signed rank test, p<0.05). Plan quality for these plans can potentially be improved by introducing more beam angles.

Conclusion Hypofractionation of LNO on the MRL is feasible, but the most suitable fractionation scheme should be investigated by pre-treatment imaging and planning. The optimal fractionation scheme differs per patient and is dependable on the presence of OARs near the target. For single fraction treatment, in which OAR constraints are more limiting, pre-treatment planning alone is not an optimal indicator for the online daily situation. [1] D. Winkel et al. Adaptive radiotherapy: The Elekta Unity MR-linac concept. ctRO 2019;18:54-59. [2] S. Chmura et al. A Phase 1 Study of Stereotactic Body