ESTRO 2022 - Abstract Book

S167

Abstract book

ESTRO 2022

Results The average delineated tumour volume was significantly higher when delineated on T1gd (0.57 cm 3 ) compared to the delineations on T2 (0.51 cm 3 , p = 0.01). The average interobserver variation appeared slightly higher on T1gd (0.41 mm) compared to T2 (0.35 mm), although the difference was not significant (p=0.12). Analysing the separate regions, significant higher variations were found at the edge of the tumour compared to sclera and vitreous. Based on the individual delineations, the source of these higher local SDs at the edge was identified as variance in whether choroidal enhancement was included in the GTV or not. We also found that the contrast between retinal detachment and tumour on T2 was low in some patients, which underscores the importance of side-by-side evaluation of all available sequences.

Conclusion The interobserver variation on T1gd (0.41 mm) and T2 (0.35 mm) are low with respect to the voxel size. Higher inter- observer variations were found at the edge of the tumour. However, these might be resolved with clear guidelines and after histopathological validation of the underlying reason for choroidal enhancement at the tumour edge. We recommend to delineate based on the T1gd-weighted scans, as parts of the tumour might be underestimated on T2.

MO-0212 Dosimetric and geometric impact of on-patient placement of air coils in a clinical MRI-only workflow

J. Scherman 1 , S. af Wetterstedt 1 , E. Persson 1 , L.E. Olsson 1 , C. Jamtheim Gustavsson 1

1 Skane University Hospital, Department of Radiation Physics, Lund, Sweden