ESTRO 37 Abstract book

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ESTRO 37

secondary organoids formation efficiency when compared to wild-type mice/untreated SG (p<0.05). However, neither endogenous nor exogenous GDNF increased the CD24 hi CD29 hi stem cell-like population. Organoids IR surviving fraction (OSF) of mSGSCs were similar albeit resulted in larger spheres and an increased cell number in the Gdnf wt/hyper compared to Gdnf wt/wt group. Indeed, mSGSC of Gdnf wt/hyper mice showed high sphere forming efficiency upon replating. Cell cycle distribution of SGSC after irradiation had no difference with or without GDNF. Interestingly, GDNF expression was highly upregulated after irradiation both in vitro (p=0.008) and in vivo (p<0.05) but normalized in vivo after mSGSC transplantation-related regeneration. Receptor tyrosine kinase (RET) expression was strongly related to OSF (r=0.842, p=0.002) and co-localized with GDNF in salivary gland striated ducts. More than 90% IR induced senescence located in striated ducts where GDNF colocalized with senescence marker p16 and SA-ß-Gal. Interestingly, senescence cells were detected in organoids after IR. Conclusion GDNF does not protect mSGSCs against irradiation but seems to promote mSGSCs proliferation through the GDNF-RET signaling pathway. Post-transplantation stimulation of GDNF/RET pathway may enhance the regenerative potential of mSGSCs. IR induced upregulation of GDNF may be involved in IR induced senescence. OC-0583 Intestinal Stem Cells: Modeling of Reserve Stem Cell Population and Volume Effect E. Bahn 1 , M. Van Heerden 2 , J. Gueulette 3 , K. Slabbert 4 , W. Shaw 2 , J. Debus 1 , M. Alber 1 1 Heidelberg University Hospital, Department of Radiation Oncology, 69120 Heidelberg, Germany 2 University of the Free State, Department of Medical Physics, 9300 Bloemfontein, South Africa 3 Université Catholique de Louvain, Radiobiology Laboratory, B-1200 Brussels, Belgium 4 iThemba LABS, Department of Radiation Biophysics, 7129 Somerset West, South Africa Purpose or Objective The stem cell (SC) population in the intestinal crypts (IC) is a field of intense research, especially with regard to the role of SC's in tissue repair. It is even more relevant in radiation-oncology as the intestine presents an organ at risk. We have recently observed a strong volume eff ect in the survival of intestinal ICs after irradiation of different lengths of externalized mouse jejunum. Here, we present a biomathematical model that reproduces this volume effect to great precision by an increase in proliferation rate for small irradiated volumes and which, applied in a meta-analysis, resolves current issues on number and radio-sensitivity of reserve SCs. Material and Methods Interpretation of the in vivo clonogenic assay is commonly obtained via a formula that relates IC survival to the number of clonogenic cells per IC and their radio- sensitivity. This IC survival model is the only one used up to date despite questionable underlying assumptions and highly inconsistent results between studies. Our model calculates the full statistics of IC procreation over time on a cell level, which allows a strict model validation on several additional experimental levels beyond the mere IC count, such as cell-population-per-IC statistics and spatial distribution of surviving ICs. Results We could successfully test the model on a large dataset: IC survival for three different irradiated volumes (whole- bowel, 10mm and 5mm jejunum) at 4-8 irradiation doses; spatial distribution of IC survival at different doses;

distribution of cell survival per IC at different doses and volumes. We observed no sign for a migration mechanism of SCs in our data, as all datasets could be reproduced by statistical variation. This might hint towards a purely stochastic repopulation mechanism in the first days of regeneration. The model reproduced precisely the volume effect on all of these levels if an increase in the PR was assumed while all other parameters remained unchanged (Fig. 1). The volume effect is observed for the 5mm, but not for the 10mm field, which indicates a threshold volume, in accordance with volume effects in other organs.

We applied this model in a meta-analysis of a large number of data from the last four decades. The results now give a consistent picture of a precise number of clonogenic cells of 31±6, which are, in contrast to prior predictions, radio-resistant (α = (0.21±0.02)Gy^-1) (Fig. 2).

Conclusion Our model dramatically increases the precision of the clonogenic assay and turns it into a precision tool for the investigation of SC procreation. It explains the small-field volume effect in intestinal epithelium as accelerated repopulation. Our experiments show that below a threshold volume, the overall proliferation rate drastically increases. The results from the meta-analysis resolve a persisting conflict in the predicted size of the reserve SC population and the radio-sensitivity of reserve SCs. This creates new opportunities for high-precision analysis of e.g. the effects of biochemical agents. OC-0584 Acute changes in leukocyte populations driving fibrosis following focal irradiation of the intestine I. Verginadis 1 , B. Bell 1 , S. Koduri 1 , C. Koumenis 1 1 Perelman School of Medicine- University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA