ESTRO 2023 - Abstract Book

S1685

Digital Posters

ESTRO 2023

Results In target V95% coverage we obtained the largest differences when the insert in target was changed. Replacing air with water density resulted in an increase of 5,8±1,1% while substituting the water insert with air decreased the coverage by 9,8±1,1%. Variation is due to the different techniques. The presence or absence of OAR gas did not affect the target V95% more than 0,5 %. The same trends were obtained for target Dmean (Figure 1.). V100% of the target (PTV) showed the largest difference when insert was changed from water to air in the target (-20,5±5,3%), and change in OAR insert for VMAT also resulted in a large decrease for target coverage (-14%). Mean dose to nearby OAR inserts changed less than 2% for any set up except for insert 5 when the insert was removed. The difference increased to more than 10% when original optimisation aimed to lower the mean dose to this volume. Conclusion Online adaptation on Ethos cannot take into account appearance or disappearance of air in body cavities as a non deformation based change in anatomy. Target coverage and mean dose are most likely to deteriorate when air appears in the target while maximum dose increases the most where air is disappearing in the target. The dose to nearby OARs is less sensitive for changes in gas filling at or near the target. 1 Aalborg University Hospital, Department of Medical Physics, Oncology, Aalborg, Denmark; 2 Aalborg University, Department of Clinical Medicine, Aalborg, Denmark; 3 University of Leeds, Leeds Institute of Medical Research at St James’s, Leeds, United Kingdom; 4 St James’s University Hospital, Leeds Cancer Centre, Leeds, United Kingdom; 5 Aalborg University Hospital, Department of Radiology, Aalborg, Denmark; 6 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark Purpose or Objective There is limited data on the interfraction motion of primary rectal tumours, hindering robust PTV margin calculations. We examined in-plane surface variation of the GTV on prospectively collected MRI data using a point-based surface displacement metric, with the aim of estimating systematic and random variations for PTV margin calculation. Materials and Methods We collected MRI scans before (x3) and during (x3) radiotherapy (RT) in a prospective clinical imaging study (NCT03619668). All patients were treated for locally advanced rectal cancer (T2-4 N0-2) with long course RT (50.4 Gy/28 frac.) and concomitant chemotherapy. The MRI scans (T2-weighted on a Philips Ingenia 3T MRI) were co-registered using a rigid bony match, and GTVs delineated by an experienced oncologist. Each GTV was represented by 5000-23.000 surface points, depending on the size. The within-patient surface variation was calculated using a point-based bidirectional local distance (BLD). For each point position, the distance between the point on the baseline scan and subsequent MRI scans was calculated as a 3D displacement vector. To collate information across patients, a reference rectum structure was created, consisting of 120 equidistant surface points on each slice. Baseline GTV volumes (and their set of displacement vectors) were transferred to the reference geometry using the relative distance from the anal verge and correlated to a point using the BLD metric. Systematic ( ∑ ) and random ( σ ) variation, were calculated for each surface point on the reference rectum (see Figure 1). To evaluate anterior, posterior, and left/right directions, each reference slice was divided into four 90º angle spans (30 points each). To evaluate different regions along the height of the rectum, three sections were defined and analysed separately. Based on ∑ and σ from each section, local anisotropic PTV margins were calculated. PO-1932 Rectal tumour position variation: Systematic and random variations by point-based surface evaluation D.T. Arp 1,2 , A.L. Appelt 3,4 , M.S. Nielsen 1,2 , R. Mikalone 5 , L.Ø. Poulsen 6,2