ESTRO 2023 - Abstract Book

S549

Sunday 14 May 2023

ESTRO 2023

Conclusion Being several orders of magnitude faster than conventional methods, LaPy can be applied to speed-up 4D-CT workflows, providing accurate deformations between phases in a few tens of milliseconds. Thus, it can be highly useful for treating moving tumors, helping to derive internal target volumes or serving as a QA tool providing fast dose accumulation in charged particle treatments affected by interplay effects. PD-0658 Use of 4DMRI acquired on an MR-Linac to quantify intra-fraction motion in pancreatic cancer J. Gough 1,2 , A. Wetscherek 2 , B. Lecoeur 2 , S. Alexander 1,2 , R. Westley 1,2 , B. Ng-Cheng-Hin 1 , A. Hunt 1 , K. Aitken 1,2 1 Royal Marsden Hospital, Radiotherapy, Sutton, United Kingdom; 2 Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom Purpose or Objective Dose-escalated radiotherapy (RT) may improve survival for patients with locally advanced pancreatic cancer (LAPC)(1,2). However, accuracy of delivery can be compromised by intra-fraction motion of the target and adjacent organs at risk (OARs). MR Linacs (MRLs) are a novel platform for LAPC stereotactic body RT (SBRT) delivery, maximising accuracy through use of improved image guidance and adaptive planning thus enabling dose escalation without exceeding OAR tolerance. Abdominal compression (AC) may be used for SBRT where more advanced motion management methods e.g. tracking are not available. Current workflows utilise 4DCT to quantify residual motion and determine internal target volumes (ITVs). However, 4DCTs are acquired once prior to treatment and no further 4D imaging is routinely acquired, assuming that motion remains unchanged. This study evaluated use of 4DMRI, acquired on an Elekta Unity MR-Linac (Elekta AB, Stockholm, Sweden), to quantify intra-fraction target and OAR motion and the effects of AC. Materials and Methods Patients with LAPC undergoing RT on standard linear accelerators were recruited to undergo MRL imaging under the PRIMER (NCT02973828) study. Patients underwent scans with and without AC with a ZifixTM (Qfix,USA) belt, including balanced 3DVane sequences acquired in free breathing. The 3DVane is a volumetric radial gradient echo sequence with stack-of-stars readout and golden-angle spacing. 3DVanes were retrospectively reconstructed into 4DMRI comprised of 10 respiratory phases using an in-house implementation of XD-GRASP(3). Pancreatic gross target volumes (GTVs) and OARs were delineated on maximum inspiration and expiration 4DMRI phases. Diaphragmatic motion on 4DMRI was compared with corresponding 4DCTs without AC for each patient. Centre of mass (COM) motion coordinates of GTVs and OARs were extracted to calculate peak to peak motion in all planes and compared with and without AC. Results 10 LAPC patients were recruited and 44 3DVane sequences acquired during 22 scanning sessions. Some were not possible to reconstruct due to data corruption or missing raw data (n=8). 36 4DMRI were analysed: without AC (n=18), with AC (n=18). There was no significant difference in diaphragmatic motion between 4DCT and 4DMRI (median 11.4mm (range 4.1 28.9mm) vs. median 10.4mm (range 4.3-22.7mm) respectively, p =0.082). Target and OAR motion on 4DMRI was greatest in the superior-inferior plane and there was a reduction in motion with AC. Median superior-inferior COM motion with and without AC of GTV and OARs are shown in Table 1.

Conclusion