ESTRO 2023 - Abstract Book

S1108

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ESTRO 2023

the three G5 toxicities (perforated gastric ulcer following treatment to left adrenal) [2]. This reflects the challenge of delivering SABR to abdominal targets near the gastric wall. Chen et al. demonstrated that stereotactic MR guided adaptive radiotherapy (SMART) achieved a significant reduction in gastric normal tissue complication probability (NTCP) in the treatment of left sided adrenal lesions [3]. The aim of this study is to quantify the benefit of daily adaptation on gastric NTCP in a cohort of patients receiving SMART to abdominal OMD. Materials and Methods This was a retrospective planning study. Consecutive patients receiving SMART to abdominal OMD were identified from institutional records. Patients where any part of the stomach was included within 2cm of the PTV on the treatment plan were included. Whole stomach was outlined on the planning scan as well as treatment scans according to published atlases. Mean stomach filling and inter-fractional percentage filling difference for the treatment course was calculated. NTCP was calculated using the Lyman-Kutcher-Burman model from baseline treatment plan (baseline NTCP), baseline plan overlayed on the anatomy of the day (pre-adaptation NTCP) and from the adaptive plan (post-adaptation NTCP). Mean NTCP values were compared using signed rank test. Results Out of 50 patients receiving SMART to abdominal targets, 25 fulfilled the inclusion criteria for this retrospective planning study. Baseline demographics are summarised in Fig 1a Mean stomach filling on baseline plan was 189.6cc (±87.7). Mean percentage difference in inter-fractional gastric filling was 36.8% (±17.5) Fig 1b Mean baseline gastric NTCP for all treatments was 0.62 (95% CI 0.41-0.83); mean pre-adaptation NTCP was 5.95 (1.91-10), reduced to 0.78 (0.45-1.11) post adaptation (p=0.0015**). A clinically meaningful NTCP reduction of 20% or greater (max 38%) was demonstrated in 20% of analysed treatments.

Conclusion We demonstrate significant reduction in gastric NTCP through daily plan adaptation using SMART in the treatment of abdominal OMD.

References

1. Harrow S Int J Radiat Oncol Biol Phys. 2022 Nov 15;114(4):611-616 2. Palma DA Lancet. 2019 May 18;393(10185):2051-2058 3. Chen H Radiother Oncol. 2021 Oct; 163:14-20.

PO-1380 Evaluation of loco-regional recurrences using DIR after iHD-SBRT in localised pancreatic cancer

M. Manderlier 1 , S. Poeta 2 , J. Engelholm 3 , D. Van Gestel 1 , J. Van Laethem 4 , L. Moretti 1 , A. Gulyban 2 , C. Bouchart 1

1 HUB Institut Jules Bordet, Department of Radiation Oncology, Brussels, Belgium; 2 HUB Institut Jules Bordet, Medical Physics Department, Brussels, Belgium; 3 Hopitaux Iris Sud, Department of Radiology, Brussels, Belgium; 4 Hôpital Universitaire Erasme, Department of Gastroenterology, Hepatology and Digestive Oncology, Brussels, Belgium Purpose or Objective To perform an imaging-based clinical evaluation of the loco-regional recurrence (LRR) pattern after isotoxic high dose stereotactic body radiotherapy (iHD-SBRT) for localised pancreatic cancer using deformable image registration (DIR).