ESTRO 2025 - Abstract Book

S790

Clinical - Gynaecology

ESTRO 2025

Conclusion: Our results show that adjuvant chemotherapy in high-risk EC patients is associated with increased HT grade ≥2. Using ROC analysis we were able to identify BM dose constraints correlating with these toxicities, suggesting that the current V40 Gy threshold of <35% should be lowered to 20-25%. Additionally, we propose a new V30 constraint, recommending that it be kept below 40-45%.

Keywords: Endometrial cancer, Bone marrow Dose constraints

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Digital Poster Immune cell function during and after bone marrow sparing versus non-bone marrow sparing EBRT in cervical cancer Anouk Corbeau 1 , Marij J.P. Welters 2 , Sanne Boekestijn 2 , Jan Willem M. Mens 3 , Henrike Westerveld 3 , Mila Donker 1 , Laura A. Velema 1 , Hélène van Meir 4 , Mariëtte I.E. van Poelgeest 5 , Judith R. Kroep 2 , Ingrid A. Boere 6 , Sander C. Kuipers 3 , Jeremy Godart 3,7 , Remi A. Nout 3 , Carien L. Creutzberg 1 , Sjoerd H. van der Burg 2 , Stephanie M. de Boer 1 1 Department of Radiation Oncology, Leiden University Medical Center, Leiden, Netherlands. 2 Department of Medical Oncology, Oncode Institute, Leiden University Medical Center, Leiden, Netherlands. 3 Department of Radiotherapy, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, Netherlands. 4 Department of Gynecology, Tergooi Medical Center, Hilversum, Netherlands. 5 Department of Gynecology, Leiden University Medical Center, Leiden, Netherlands. 6 Department of Medical Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, Netherlands. 7 Department of Medical Physics & Informatics, HollandPTC, Delft, Netherlands Purpose/Objective: Primary chemoradiotherapy (CRT) suppresses the immune system in women with locally advanced cervical cancer (LACC) as collateral effect[1]. Innovative external beam radiation therapy (EBRT) techniques and bone marrow sparing (BMS) constraints could reduce bone marrow dose, therefore possibly reducing immune suppression. The aim of this study is to evaluate immune cell composition and function and compare these between non-BMS 3D conformal radiation therapy (3DCRT) or intensity-modulated radiation therapy (IMRT) and BMS volumetric modulated arc therapy (VMAT), followed by brachytherapy, for primary LACC. Material/Methods: Hematologic data were compared between eleven women treated with non-BMS 3DCRT (7/11) or IMRT (4/11) (2011 2014)[1] and eighteen women treated with BMS VMAT (2022-2024)[2] combined with weekly cisplatin (40 mg/m 2 ). For BMS, the outer contour of the pelvic bones was spared according to V 10Gy <90%, V 20Gy <65%, and V 40Gy <15-20%[3]. Blood sampling was performed at baseline and follow-up until 3 months (non-BMS group) and 12 months (BMS group) after treatment. Blood cell counts, reactivity of T cells to antigenic stimulation, antigen presenting capacity, and immune cell composition were determined as described previously[1]. Blood cell counts were analyzed using linear mixed modeling with timepoint as a fixed factor and compared between groups using unpaired t-tests with Bonferroni correction. Hematologic toxicity was classified according to CTCAEv5.0. Results: Extended field irradiation was used in 2/11 (18.2%) and 10/18 (55.6%) women in the non-BMS and BMS group respectively, with a mean pelvic bones D mean of 34.5 Gy (range 33.5–36.5 Gy) and 24.6 Gy (range 18.3-28.0 Gy). Examples of treatment plans are provided in Figure 1. Both groups showed a significant decline in number of lymphocytes, leukocytes, and neutrophils during treatment, which remained reduced compared to baseline with only slight recovery. There were no significant differences in these counts between the groups at all timepoints (lymphocytes presented in Figure 2). All women experienced lymphopenia ≥grade 2. Antigenic response, antigen