ESTRO 2022 - Abstract Book

S1266

Abstract book

ESTRO 2022

Conclusion Our current isotropic pLN margins of 13 mm for prostate based IGRT can safely be reduced. Margin reduction allows for reduction of dose to organs at risk but can also enable target dose escalation. However, margin reduction must be based on careful consideration of all uncertainties in the planning- and treatment process. Each clinic should therefore derive their own treatment margins based on locally estimated uncertainties.

PO-1490 SBRT Patient Specific QA: Adaptive dose accumulation from deformable CBCT image registration

S.B. Vestergaard 1 , M.S. Nielsen 2,3 , R. Kjeldsen 1

1 Aalborg University Hospital, Clinical Surgery and Cancer Treatment, Aalborg, Denmark; 2 Aalborg University Hospital, Clinical Surgery and Cancer Treatment, Aalbrog, Denmark; 3 Aalborg University, Clinical Medicine, Aalborg, Denmark Purpose or Objective Daily image guided radiotherapy does not necessarily handle anatomical changes concerning organs at risk (OAR). Thus, dose to the OAR may be a limiting factor for stereotactic body radiation therapy (SBRT) as the dose may exceed tolerance levels. The aim is to justify a patient specific dose plan Quality Assurance (QA) for individual treatment sessions. Materials and Methods Patient material consist of 24 patients referred for lung SBRT (3 x 22.5 Gy). A CBCT acquired prior at each treatment fraction, are matched trough a workflow starting with a bone match follow by a soft tissue (tumor) match (Online-match). As an offline strategy a solitary bony match is performed, leading to strategies with one starting from a bone match and the other from the tumor match. the three CBCTs are deformable registered to the planning CT (Velocity 4.1, Varian Medical Systems) and a ‘plan-of-the-day’ is calculated on the generated artificial-CT (aCT). The calculated dose distribution is deformed back to the original CT, then subtracted from the planed dose distribution. The dose difference for the OARs are examined. Due to OAR dose constrains defined by maximum values, only the 200 voxels (0.4 cm ³ ) in an OAR (OAR 200 ) receiving the larges additional doses are included. A total of 60 structure sets (10 patients, three CBCT and two deformations) are examined for correlation between the deform-registered and radiographer contoured OAR on each aCT. Dice-Similarity coefficient (DSC) and Hausdorff distance (HD) are calculated, in order to ensure the quality of the deformable registrations. Results The association between the deform-registered and radiographer contoured structures (fig. 1) shows mean DSC ranging [0.92; 0.98] and the mean HD ranging [0.06; 0.10] cm. The 95% percentile HD ranging from [0.12; 0.38] cm, with deviations primarily from the esophagus and heart due to broad distributions. The added mean dose to OAR 200 (fig. 2) ranging from 0.5 Gy for the trachea and up to 5 Gy for the aorta. The maximum dose difference for a single OARs is 18 Gy for the aorta. When comparing OAR 200 dose difference for the two match strategies it is see that on average OAR 200 receive more [0.1-1.1 Gy] with Online-match then with Bone-match.