ESTRO 2022 - Abstract Book

S452

Abstract book

ESTRO 2022

Radiumhospital, Oslo University Hospital, Department of Oncology, Oslo, Norway; 6 Institute of Oncology Ljubljana, Department of Radiation Oncology, Ljubljana, Slovenia; 7 Tata Memorial Center, Department of Radiation Oncology, Mumbai, India; 8 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 9 Academisch Medisch Centrum , Department of Radiation Oncology, Amsterdam, The Netherlands; 10 Cross Cancer Institute, University of Alberta, Department of Oncology, Edmonton, Canada; 11 Leiden University Medical Center, Department of Clinical Oncology, Leiden, The Netherlands; 12 Addenbrooke’s NHS Trust, Department of Oncology, Cambridge, United Kingdom; 13 North Estonia Medical Centre Foundation, Department of Oncology, Tallin, Estonia; 14 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective To simulate the effect of dose de-escalation and compressed EBRT and/or brachytherapy (BT) fractionation schedules on target and organ doses in low-risk cervix cancer. Materials and Methods 1261 patients enrolled in the EMBRACE II study were treated with 45Gy/25fx EBRT and MRI-guided BT. 312 (25%) node positive and negative patients were identified as having low-risk of local failure based on histology (squamous cell) and extent of primary tumour (T-score ≤ 4 [1]) and used for further analysis. EBRT of 45Gy/25fx and 40Gy/20fx were combined with different BT fractionation schedules. For HDR: a) BT fractionation as applied clinically (“clinical”), b) 3fx, c) 2fx. For PDR: a) “clinical”, b) 1fx of 24 pulses. Target and OARs doses were simulated for CTV HR D90% normalised to 90Gy and 85Gy EQD2 10 . Differences in targets/OARs doses between modified and clinical schedules and adherence to hard/soft EMBRACE II OAR constraints [2] was evaluated. Results Median (IQR) tumour maximum dimension at diagnosis was 38 (32-47)mm, and CTV HR volume at BT was 23 (18-31)cm 3 . Simulated target/OAR doses and the percentage of patients adhering to all OARs hard/soft constraints are presented in Table1. For clinical EBRT/BT plans, mean CTV HR D 90% was 93Gy EQD2 10 . Limiting CTV HR D 90% to 90Gy (85Gy) for clinical fractionation schedules decreased OAR doses by 1-2Gy (2-5Gy), respectively, with most prominent effect seen in bladder D2cm 3 . Compressed BT with 3fx HDR or 1fx PDR delivering 90Gy to CTV HR D 90% maintained on average similar OARs doses as clinical plans. However, global adherence to all OARs soft/hard constraints decreased by 7%/11%. Further CTV HR dose de- escalation to 85Gy decreased OAR doses by 2-3.3Gy and nudged adherence to soft/hard constraints by 12%/-2%. With additional BT hypofractionation to 2fx HDR and 85Gy normalization, adherence to hard constraints dropped by 5% while physical CTV HR D90% were high, >11Gy. EBRT of 40Gy/20fx with clinical or compressed BT schedules resulted in overall reduction in OAR doses of 0-2Gy, compared to 45Gy/25fx with corresponding BT schedules. A small percentage of patients had increased OAR doses, bladder mainly, for the 40Gy/20 EBRT schedules. Generally, CTV IR D98% was <60Gy, mainly with 85Gy normalization and/or 40Gy/20fx EBRT.

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