ESTRO 2021 Abstract Book

S338

ESTRO 2021

MSCs maintained their stem cell traits after exposure to different types of particle ions with clinically relevant doses. Considering the preserved differentiation ability, low apoptosis rates and effective DNA repair, MSCs should be further evaluated as potential stem-cell based therapy for particle ion-related toxicities.

PH-0439 An innovative strategy in cancer treatment: Proton minibeam radiation therapy T. Schmid 1,2 , A. Dombrowsky 3 , M. Sammer 4 , J. Reindl 5 , G. Dollinger 4 , S. Bartzsch 3 , S. Combs 6 1 Klinikum rechts der Isar, Technische Universitaet Muenchen, Department of Radiation Oncology, Muenchen, Germany; 2 Helmholtz Zentrum München, Institute of Radiation Medicine (IRM), Neuherberg, Germany; 3 Helmholtz Zentrum München, Institute of Radiation Medicine (IRM) , Neuherberg, Germany; 4 Universität der Bundeswehr, Institute for Applied Physics and Metrology, Neubiberg, Germany; 5 Universität der Bundeswehr,, Institute for Applied Physics and Metrology , Neubiberg, Germany; 6 Klinikum rechts der Isar, Technische Universitaet Muenchen, Department of Radiation Oncology , Muenchen, Germany Purpose or Objective Normal tissue complications are a common side effect after conventional radiotherapy. Proton minibeam radiotherapy (pMBRT) is a spatial fractionation method that widens the therapeutic window. By using submillimeter proton beams the treatment dose can be modulated, known as spatial fractionation. This is based on the dose-volume effect, which says that the tissue tolerance increases for smaller irradiation fields. The aim of this study was to analyze if normal tissue complications depend on the irradiation accuracies of a radiation treatment combining spatial fractionation with temporal fractionation. Materials and Methods Four fractions of 20 MeV pencil proton minibeams with a size of σ = 222±5 μm were delivered daily to healthy ears of Balb/c mice in three different schemes. A total of 6 to 8 mice were assigned to each group. Ears were positioned in each fraction according to the characteristic blood vessel structure achieving an average position accuracy of 110±52 μm. Acute toxicity (ear swelling) was evaluated in a period of 150 days. On day 150, fibrosis, as a late toxicity, was semi-quantitatively analyzed by Sirius red staining of collagen. Results A pattern of 16 minibeams with a center-to-center (ctc) distance of 1.8 mm was delivered to the same position in every fraction and induced the lowest swelling while a pattern shift by ctc/2 between the fractions led to a significant stronger acute reaction. The highest increase in ear thickness and additionally in fibrosis induction was measured when 64 minibeams with a ctc of 0.9 mm were irradiated. At the end of the follow- up, the ears which were irradiated four times at the same position were 265.8±3.6 μm thick which was not different to the sham-irradiated ears (266.1±7.2 μm). These results demonstrate that the normal tissue response depends largely on the spatial dose distribution of the minibeam pattern. Conclusion In conclusion, both acute and late normal tissue complications depend on the accuracy of the reirradiation positon of proton minibeams. By using a high reirradiation accuracy, ear swelling and the amount of fibrotic tissue were almost on the same level as sham-irradiated mice. However, a lack of accuracy slightly increased the normal tissue complications when a highly dose modulated minibeam pattern is applied. PH-0440 Investigating The Role of the cGAS-STING Pathway in Mediating Radiation Induced Bystander Effects P. Cahoon 1 , S.J. McMahon 1 , K. Prise 1 1 Queen's University Belfast, Patrick G Johnson Centre for Cancer Research, Belfast, United Kingdom Purpose or Objective Radiotherapy is typically understood in terms of the direct effects of radiation on cells. However, there is an increasing focus on non-targeted effects (NTEs) of radiation, such as Radiation-Induced Bystander Effects (RIBEs) and the abscopal effect. RIBEs are the phenomenon of unirradiated cells exhibiting a response to irradiated neighbours, while abscopal effects relate to radiation’s ability to provoke a response in vivo in unirradiated sites distant from the irradiated tumour, and are believed to be immune-driven. However, despite recent interest, the mechanisms governing NTEs remains elusive. Cytoplasmic dsDNA is known to play a role in activation of the cGAS-STING pathway, a key driver of immune responses, and has also recently been implicated as a mediator of RIBEs. This research aimed to investigate if these two seemingly distinct processes share a common underlying pathway. Materials and Methods Clonogenic survival assays were used to investigate cell survival in and out-of-field under different irradiation conditions. The impact of removing dsDNA was investigated by treating MyC-CaP mouse prostate cancer cells with DNase I at various timepoints post irradiation. The involvement of the cGAS-STING pathway was further investigated by repeating these experiments in MyC-CaP derived cell lines with either cGAS or STING knocked out. Immunofluorescence staining with anti-dsDNA antibody was utilised to quantify cytosolic dsDNA present in both directly irradiated and bystander cells. Co-culture assays were used to investigate how incubating parental MyC-CaPs cells and STING knockout cells together influenced clonogenic survival. In these experiments one cell line was irradiated directly while the other was treated as an out-of-field bystander line. Results Both removing the damage signal (by degrading dsDNA) and inhibiting the sensing of this signal (by knocking out cGAS or STING) effectively abrogated the out-of-field cell killing associated with the RIBE. No additive effect was seen by combining DNase I treatment with cGAS or STING knockout. Immunofluorescence staining confirmed that cytosolic dsDNA increased within irradiated and bystander cells following irradiation.