ESTRO 2021 Abstract Book

S84

ESTRO 2021

della Scienza, Department of Medical Physics, Turin, Italy; 5 AOU Città della Salute e della Scienza, Department of Medical Physics, Turin, Italy

Purpose or Objective We investigated the role of the selective avoidance of hematopoietically active pelvic bone marrow (BM), with a targeted intensity-modulated radiotherapy (IMRT) approach, to reduce acute hematologic toxicity (HT) in anal cancer patients undergoing concurrent chemo-radiation Materials and Methods We designed a one-armed two-stage Simon’s design to test the hypothesis that BM-sparing IMRT would improve by 20% the rate of G0–G2 (vs G3-G4) HT, from 42% of RTOG 0529 historical data to 62% (α=0.05;β=0.20). A minimum of 21/39 (54%) with G0-G2 toxicity represented the threshold for the fulfilment of the criteria to define this approach as ‘promising’. We employed 18 FDG-PET to identify active BM within the pelvis. Acute HT was assessed via weekly blood counts and scored as per the Common Toxicity Criteria for Adverse Effects version 4.0 Results From December 2017 to October 2020, we enrolled 39 patients. Maximum observed acute HT comprised 20% rate of >G3 leukopenia and 11% rate of >G3 thrombocytopenia. Overall 11 out of 39 treated patients (28%) experienced >G3 acute HT. Conversely, in 28 patients (72%) G0-G2 HT events were observed, above the threshold set (Figure). No significant differences were found between patients experiencing G3-G4 acute HT and those who did not in terms of age, gender, tumor and nodal stage and most of the dosimetric parameters employed during the optimization process ( ACT PBM V 10 , ACT PBM V 20 and ACT LSBM mean dose). A borderline statistical significant difference (p=0.055) was observed in terms of ACT LSBM V 40 . Specifically mean ACT LSBM V 40 was found to be 24.8 Gy (SD: 7.8) for patients not experiencing major acute HT and 30.4 Gy (SD: 9.8) for those in which G3-G4 acute HT was observed.

Conclusion Hence, 18 FDG-PET-guided BM-sparing IMRT was able to reduce acute HT in anal cancer patients undergoing concurrent chemoradiation with definitive intent.

PH-0116 Zebrafish avatars as radiosensitivity predictors in Rectal Cancer: towards personalized treatment O. Parés i Grau 1 , B. Costa 2 , S. Vieira 1 , J. Stroom 1 , M.J. Cardoso 1 , R. Coutinho 1 , R. Rio-Tinto 3 , M. Bispo 3 , I. Santiago 4 , L. Fernandez 5 , N. Figueiredo 6 , C. Greco 1 , R. Fior 7 1 Champalimaud Foundation, Radiation Oncology Department, Lisbon, Portugal; 2 Champalimaud Foundation, Cancer Development and Immune Evasion Lab., Lisbon, Portugal; 3 Champalimaud Foundation, Gastroenterology, Digestive Unit, Lisbon, Portugal; 4 Champalimaud Foundation, Radiology Department, Lisbon, Portugal; 5 Champalimaud Foundation, Colorectal Surgery, Digestive Unit , Lisbon, Portugal; 6 Lusíadas Hospital, Surgery Department, Lisbon, Portugal; 7 Champalimaud Foundation, Cancer Development and Immune Evasion Lab. , Lisbon, Portugal Purpose or Objective Response to radiotherapy in locally advanced rectal cancer (LARC) is highly heterogeneous and predictive biomarkers of response before treatment could be of major importance for treatment decision-making. The aim of this study is to present preliminary results on zebrafish Avatars as a fast, in vivo model for radiosensitivity prediction in patients with LARC. Materials and Methods Patients with a newly diagnosed LARC proposed for neoadjuvant RT, were offered to participate in this study. Prior to treatment, fresh tumor tissue was collected via endoscopy-guided macrobiopsy and divided for (1) pathology assessment and (2) microinjection into zebrafish larvae to generate patient-derived xenografts (zAvatars). Viable zAvatars were irradiated with a 6MV X-ray beam to a dose of 1x25Gy using a linear accelerator. Two days after RT, levels of Caspase3 as a surrogate of apoptosis were accessed by confocal