ESTRO 2020 Abstract Book

S198 ESTRO 2020

PH-0367 Evolution of the tumour extent during long course radiotherapy for glioblastoma patients R. Dahlrot 1,2 , B. Axelsen 3 , M. Klüver-Kristensen 3 , F. Mahmood 2,3 , J.T. Asmussen 4 , O. Hansen 1,2 , A. Bertelsen 3 , S. Hansen 1,2 , C. Brink 2,3 , U. Bernchou 2,3 1 Odense University Hospital, Department of Oncology, Odense C, Denmark ; 2 University of Southern Denmark, Department of Clinical Research, Odense, Denmark ; 3 Odense University Hospital, Laboratory of Radiation Physics, Odense C, Denmark ; 4 Odense University Hospital, Department of Radiology, Odense C, Denmark Purpose or Objective Intracranial changes occurring during radiotherapy (RT) in patients with glioblastoma (GBM) are difficult to detect using the standard CT based image guidance techniques available at most linear accelerators. Critical increase in the tumour extent may therefore be missed which potentially could lead to insufficient target dose coverage. This study used MRI, known to provide much better soft- tissue contrast, in a longitudinal setting to investigate the changes in tumour extent throughout the RT course in GBM patients. Material and Methods The prospective study included 13 GBM patients referred for 59.4 Gy in 33 fractions. Gadolinium contrast enhanced T1-weighted MRI scans (1x1x2 mm 3 voxel size) were performed for RT planning, on fraction 10, 20, 30 (±2), and four weeks after the end of RT on the same Philips Ingenia 1.5 T system. For RT planning, the original gross tumour volume (GTV Org ) was defined as the postoperative residual tumour including the surgical cavity. The clinical target volume (CTV) was subsequently constructed using a 20 mm isotropic expansion of the GTV, reduced at anatomical barriers and at relevant organs at risk. The planning target volume was created from the CTV by addition of a 3 mm margin. All MR scans were fused with the planning CT scan and the additional GTV re-delineated (GTV Re ) were constructed– resulting in four new GTV Re per patient. Volume ratio of GTV Re and GTV Org were calculated. Furthermore, the maximum distance from any point on the GTV Re contour to the closest point on the GTV Org contour was measured – considering only GTV Re extending outside GTV Org . Results Example slices including GTV Org and GTV Re delineations in MRI scans of the patient with the relative largest tumour growth during RT is shown in figure 1. The relative GTV volume is plotted for the different time points for all cases in figure 2A. Most cases display monotonic or little change in GTV volume over time. The maximum distance of GTV Re outside GTV Org is shown for all cases in figure 2B. For most cases the largest part of change in tumour extent has occurred at the 10 th fraction. Only the case presented in figure 1 had GTV Re contours more than 20 mm from GTV and thus resided outside the CTV during part of the RT treatment course.

Conclusion Large variation in the tumour extent may occur during a long course RT for GBM patients. However, this small study suggests that most changes take place within the first two