ESTRO 2023 - Abstract Book

S504

Sunday 14 May 2023

ESTRO 2023

this multi-institutional analysis was to compare different software solutions and implementations of DDA and investigate differences in resulting dose metrics. Materials and Methods Six institutes participated in this DDA analysis. Five data sets of patients which were treated with online adaptive stereotactic MRgRT were included: (I) cervix (5 x 7 Gy), (II) liver (5 x 7 Gy), (III) lymph node (5 x 6 Gy) and (VI,V) two prostate (5 x 7.5 Gy) cases. The T2w MRI for each fraction was re-contoured offline by an experienced radiation oncologist. Each institute performed a retrospective DDA using the software implementation available at their respective centre. First, a deformable image registration (DIR) was carried out, in which the MRIs of each fraction were deformably registered to a reference MRI (fraction 1). Three institutes used hybrid intensity/structure (A,B,C), one institute used contour (D), one intensity (E), and one normalised gradient fields/structure (F) based DIR algorithms. DIR quality was evaluated using dice similarity coefficients (DSC). Then, DDA was performed accordingly. Five institutes used direct dose mapping (A-E), one the energy mass transfer method (F). Resulting accumulated dose distributions were analysed with respect to the treating institution’s clinical dosimetric constraints (CDCs). Results Analysis of DSC indicates better DIR quality for the hybrid intensity/structure-based algorithms especially for contours that were used as guidance. The DDA dose-volume histograms (DVH) per case are shown in figure 1. In general, good agreement was found between the different DDA approaches. The dosimetric results are summarised in table 1. For case (I), rectum D2ccm showed the largest range in OAR dose metrics with 2.8 Gy. Duodenum with Dmax 7.1 Gy showed the largest OAR CDC difference for the case (II). For case (III), D0.5ccm for rectum resulted in the largest OAR CDC difference with 4.0 Gy. The greatest CDC OAR difference was observed for bladder V28Gy with 10.2 Gy and 7.6Gy for the cases (VI,V), respectively.