Effects of different DNA damaging agents have been well characterized in human cells in vitro, but little is known about the kinetics of DDR in in vivo. This is due to the fact that repeated sampling of tissue is difficult. Herein, fine needle aspirate (FNA) technique was adapted as a minimally invasive sampling method to address cellular response to DNA damaging agents in vivo (dogs). Investigated end-points are quantification of induced DNA damage, time course (kinetics) of damage formation and repair, residual damage, and functionality of specific DNA repair pathways. Tumor samples will repeatedly collected from canine patients before and after radiotherapy. The inclusion of 20 dogs per tumor type (soft tissue sarcoma, malignant melanoma, carcinoma of head & neck) is planned. Alkaline Comet assay and ?H2AX/53BP1-foci immunofluorescence are used to detect DNA damage. As preliminary work, the assays have been set up and work in dog tissue/cells. The collaborative aspect of this project consists of analyzing the in vivo results by a mathematical model that will thereby be refined by integrating the biological findings in vivo of this project. An existing and working model focusing on the difference between homologous recombination and non-homologous end-joining repair can be used as starting point. This will lead to a more comprehensive framework for designing new therapeutic approaches for anti-cancer treatment will be available. There will be a clear added value obtained by the interaction of biological experimentation, state-of-the-art data analysis and modelling / computer simulation during the whole project.