ESTRO 2021 Abstract Book

S337

ESTRO 2021

Newcastle upon Tyne, Institute of Genetic Medicine , England, United Kingdom; 6 University of Manchester, Division of Pharmacy and Optometry, England, United Kingdom; 7 University of Manchester, Department of Radiotherapy Related Research, England, United Kingdom Purpose or Objective Radiation cardiotoxicity affects one third of patients with intrathoracic cancer and is largely characterised by vascular, electrical and pump dysfunction in the months-years following radical doses. The heart is composed of a diverse range of specialised tissues but recent clinical studies suggest that the dose received by the superior region has the highest correlation with post-treatment morbidity and mortality. This region contains several critical substructures, including the proximal coronary arteries and the sinoatrial and atrioventricular nodes. In this study, we investigated the longitudinal effects of radiation on the cardiac conduction system in a partial heart irradiation mouse model and characterised the transcriptional, histological and functional changes following base irradiation. Materials and Methods 8-week old C57BL/6 mice were irradiated with a single fraction of 16Gy to the cardiac base, middle or apex of the heart using a small animal radiotherapy research platform. The base was defined as the caudal third of the heart volume on cone-beam CT, encompassing both atria and both conduction system nodes. Electrocardiography was performed at baseline and 10-week intervals for one year. Mice aged 9 months similarly underwent 16Gy/1# base irradiation, and had electrocardiography and tissue collection at baseline and 30 weeks. Paraffin-embedded tissues were stained with haematoxylin and eosin and Masson’s trichrome, and immunohistochemistry was performed for cell surface markers for endothelium, leucocytes and conduction system cardiomyocytes. Whole-organ spatial transcriptomics was used to characterise regional variations in gene expression. Results Time-dependent increases in the P wave duration and PR interval were observed in young base-irradiated mice, most apparent at 20 weeks post-irradiation. Changes resolved at 40 and 50 weeks. Similarly, P wave duration and PR interval were increased at 30 weeks in aged mice. QRS duration was not prolonged in any groups. Atrial cardiomyocyte density and myocardial fibrosis were not increased compared with controls at 30 weeks in the aged mice. Microvascular density and white cell infiltration were comparable between irradiated and control aged groups. Spatial transcriptomic analysis revealed distinct clusters of gene expression unique to the control and irradiated animals. Conclusion The cardiac conduction system exhibits subacute and self-limiting dysfunction independent of fibrosis following irradiation of the sinoatrial and atrioventricular nodes. Differential effects of radiation were identified on expression of genes linked to cardiac conduction and other cardiomyocyte functions. Further work is planned to establish the radiobiology underpinning the specialised cardiomyocytes of the conduction system nodes. This research has potential implications for radiation oncologists treating both thoracic malignancies and refractory tachyarrhythmias. PH-0438 Effects of particle irradiation on human mesenchymal stromal cells A. Rühle 1 , P. Dai 2 , R. Lopez Perez 2 , M. Strack 1 , S. Brons 3 , J. Debus 4 , P. Wuchter 5 , A. Grosu 1 , P. Huber 2 , N. Nicolay 1 1 University Medical Center Freiburg, Department of Radiation Oncology, Freiburg, Germany; 2 German Cancer Research Center (dkfz), Department of Molecular Radiation Oncology, Heidelberg, Germany; 3 Heidelberg University Hospital, Heidelberg Ion-Beam Therapy Center, Heidelberg, Germany; 4 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany; 5 Medical Faculty Mannheim, Heidelberg University, Institute of Transfusion Medicine and Immunology, Heidelberg, Germany Purpose or Objective Mesenchymal stromal cells (MSCs) have been shown to maintain their stem cell characteristics after photon irradiation. However, whether the radioresistant phenotype is also valid regarding particle irradiation, is widely unknown. Materials and Methods MSCs derived from the bone marrow of healthy donors and dermal fibroblasts (HS68) were irradiated with different types of particles, including protons (1H), helium (4He), carbon (12C) and oxygen (16O) ions at the Heidelberg Ion Therapy Center. Using photon irradiation as reference, the relative biological effectiveness was calculated at 10% clonogenic survival. The influence of the different particle ions on MSC surface marker expression as well as on the adipogenic, osteogenic and chondrogenic differentiation was examined. Cell cycle distribution and apoptosis were determined with flow cytometry measurements, and γH2AX foci analyses were performed based on immunofluorescence stainings with automatic foci quantification. Western blots were carried out to investigate the expression of proteins involved in DNA repair and cell cycle regulation. Results RBE values amounted to 1.0-1.1 for 1H, 1.9-2.3 for 4He, 2.9-3.2 for 12C and 2.6-3.1 for 16O, and were thereby higher than for HS68 fibroblasts (1.0 for 1H, 1.4 for 4He, 1.9 for 12C and 2.1 for 16O). Expression of the characteristic surface markers CD73, CD90 and CD105 as well as the differentiation ability were found unaltered after exposure to 2 Gy photons or the equivalent particle ion doses. There was a small G2/M arrest at 96h after 2 Gy photons or the biologically equivalent doses in MSC1 and MSC2. Apoptosis rates remained low and did not exceed 10% after 96h following photon (2 Gy) or particle ion (equivalent biological dose) irradiation. The γH2AX foci number/nucleus was comparable to unirradiated controls at 24h after irradiation with either 2 Gy photons or the biologically equivalent particle ion doses. In line with this finding, expression of phospho-ATM and phospho-chk2 were increased at 2h after irradiation but returned to baseline levels after 24h. Conclusion Although RBE values were found higher for MSCs than for dermal fibroblasts, human bone marrow-derived