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Introduction: X-ray microbeam radiation therapy (MRT) as a novel tumor treatment strategy deposits high doses in spatially fractionated X-ray beamlets promising reduced normal tissue toxicity, compared to conventional irradiation, and a better tumor control. Radiobiological studies of MRT with kilovoltage X-rays are mainly performed at synchrotron radiation facilities with high costs and space requirements. The Munich Compact Light Source (MuCLS) is a laboratory-sized and cost-effective source based on inverse Compton scattering of infrared laser photons [1]. Currently the most widely accepted method for assessment of treatment efficiencies is tumor growth delay with subcutaneous tumors in the hind leg of small animals. However, a new model is required for MRT with kilovoltage X-ray beams which only allow for short penetration depths. Therefore, we successfully developed a setup for a growth delay study in a tumor-bearing mouse ear model for investigation of MRT at the MuCLS. In addition, we successfully established a protocol to isolate tumor cells from irradiated tumors for evaluation of radiobiological effect on cellular level. Materials & Methods: The dose rate of the MuCLS was improved with the installation of a polycapillary collimation optic. A W-Air collimator was inserted to get a collimated X-ray beam with 50 μm wide microbeams and a center-to-center distance of 350 μm. We implemented the mouse ear tumor model with a human head and neck cancer cell line FaDu [2] suspended in extracellular matrix and subcutaneously injected into the right ear of NMRI (nu/nu) mice. After reaching a size of 2x2 mm2 tumors were irradiated using doses of either 3 and 5 Gy with 25 keV X-rays at the MuCLS. Tumor growth delay was determined with a caliper over a follow-up period of 30 days and compared between MRT, homogeneous and control mice. Animals were sacrificed when tumors reached the 15-fold initial volume. A single tumor cell suspension was prepared from excised tumors for in-vitro studies. The analysis of radiosensitivity by colony formation assay and stable chromosomal aberrations by two-color fluorescence in-situ hybridization is still on-going. Results: We successfully installed a setup at the MuCLS which allows irradiation of tumors in small animals and implemented a xenograft tumor model in mouse ears. Homogeneously irradiated tumors showed a growth delay at 5 Gy compared to control mice. There was no tumor growth delay after MRT and homogeneous irradiation at 3 Gy. Tumor cells from irradiated tumors were successfully isolated and cultured. Preliminary data shows an increased radiosensitivity of tumor cells originating from homogeneously and MRT-irradiated tumors compared to control tumor cells. Conclusion: This innovative approach allowed the irradiation of tumors in a mouse ear model at a novel laser-based X-ray source, the MuCLS. Homogeneous irradiation at MuCLS induced a tumor growth delay at 5 Gy. In addition, we successfully validated a protocol for tumor cell isolation for investigations of radiation-induced effects. Supported by the DFG Cluster of Excellence: Munich-Centre for Advanced Photonics. References: [1] Eggl et al., J. Synchrotron Rad. (2016) 23: 1137 [2] Beyreuther et al., PLos One (2017)

Introduction: The tumor growth delay assay is a well-accepted technique in experimental animal tumor models for the measurement of the response to treatments. Tumor growth delay assays were mostly performed with subcutaneous xenograft tumors in the hind leg of small animals. However, some radiation qualities with low energy and/or very small irradiation fields cannot use this method. This study was performed to test a new irradiation setup at the Small Animal Radiation Research Platform (SARRP, Xtrahl Ltd.) which can be especially used to irradiate very small tumors with low energy X-rays. Methods: This study was performed with a human head and neck cancer cell line (FaDu). 100 000 FaDu cells were suspended in Matrigel® and subcutaneously injected at the right ear of immunocompromised NMRI nu/nu mice. Tumors with a size of 2x2 mm2 were irradiated with 3 Gy and 6 Gy operating the SARRP at 70 kVp X-rays. Tumor growth was determined over a follow-up of 20 days with a caliper. The tumor growth delay was compared between homogeneously and non-irradiated mice. 20 days after irradiation tumor cells were transferred in cell culture. Results: In this pilot study using 70 kVp X-rays, six tumor-bearing mice were irradiated with either 3 or 6 Gy. Three tumor-bearing mice served as a control. The tumor volume doubling time of unirradiated tumors was 2.75 ± 0.4 days. Out of three, one mouse showed an obvious tumor growth delay at 3 Gy. However, all tumors irradiated with 6 Gy were controlled. The tumor cells which were transferred into cell culture medium showed normal growth characteristics. Conclusion and Outlook: We successfully implemented a xenograft tumor system in mouse ears and irradiations of 2x2 mm2 tumors at the SARRP. The mouse ear tumor model allows an accurate and simple method to determine the tumor volume. In future, this tumor-bearing mouse ear model will enable irradiations which are limited due to small irradiation fields and/or low X-ray energies. Moreover, it is possible to isolate tumor cells out of the mouse ear for future in-vitro analysis. This new method could be used at the first brilliant and compact synchrotron X-ray source (Munich Compact Light Source) where the dose can be deposited by spatially fractionated X-ray beamlets like microbeam radiation therapy (MRT). Acknowledgements: This work was supported by the DFG-Cluster of Excellence ‘‘Munich-Centre for Advanced Photonics’’.