ESTRO 2025 - Abstract Book

S3937

Radiobiology - Normal tissue radiobiology

ESTRO 2025

Purpose/Objective: Proton therapy is an advanced technique that precisely targets tumors while sparing surrounding tissue, offering significant benefits in cancer treatment. However, patients may still experience delayed effects, such as radiation induced brain damage leading to contrast enhancement leakage in diagnostic magnetic resonance imaging (MRI). Utilizing a preclinical model that mimics this clinical observation, we studied microglia, the brain's resident immune cells. We combined automated image-analyses of individual cells in whole-brain histological slices with calculated dose distributions and MRI changes to quantify neuroinflammation in mice undergoing precise proton irradiation of a subvolume of the brain. Material/Methods: The right hippocampal region of C3H/HeNRJ and C57BL/6JRj mice received single proton doses of 40-85 Gy in the Bragg peak [1]. To analyze late side effects, mice were followed-up for a period of six months, unless they reached a humane endpoint. Before euthanization, pre-sacrifice MRI were acquired to define the volume of contrast-enhancing lesions. The extracted brains were sectioned into 3 µm thick slices at 100 µm intervals and stained for Iba1 (microglia) and Ki67 (proliferation). The histological data were co-registered to the dose distributions calculated on cone-beam computed tomography (CBCT) images [2]. Automated microglia segmentation was applied to analyze density, activation, and proliferation on an individual cell level and correlated to dose and brain sub-regions. Microglia activation was calculated through the M-score that is based on the area and circularity of each cell [3]. Results: Microglia density shows a strong correlation with the volume of contrast-enhancing lesions, reaching statistical significance (p < 0.001) in both the C57BL/6 and C3H/He mice. A similar correlation (p < 0.001) is observed between the density of proliferating microglia and contrast-enhancing lesion volume in the analyzed C57BL/6 mice. Spatial mapping of microglial activation reveals a gradual increase in the irradiated region in mice exposed to lower doses, and a marked rise in those receiving higher doses. Activated microglia are most concentrated in the periventricular region and in areas receiving the maximum dose. All contoured subregions display a dose-dependent increase of microglia density; however, the most significant differences are detected in the cortex, which exhibits the flattest slope. In contrast, the periventricular zones, despite receiving lower total doses, show significantly steeper slopes compared to all other regions. Conclusion: Slice-based histo-cytometry enabled quantitative and spatial analyses of microglia response, which was shown to be dose- and region-dependent. Our data support microglia as a potential predictive marker and treatment target for radiation-induced brain damage.

Keywords: proton irradiation, mouse model, brain damage

References: 1. Suckert, T. et al., Front Oncol 2021; 10: 1–17

2. Soltwedel et al. Radiother Oncol 2023; 182: 109591 3. Waller, R. et al., PLoS ONE. , 2019;14(1):e0210888

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Digital Poster Cellular responses in murine salivary glands to fractionated irradiation: implications for fibrosis and hyposalivation Inga Solgård Juvkam 1,2 , Olga Zlygosteva 3 , Eirik Malinen 1,3 , Nina Jeppesen Edin 3 , Hilde Kanli Galtung 2 , Tine Merete Søland 2,4