Context: The ARRONAX cyclotron preclinical platform was upgraded to enable irradiations of mice with 70 MeV proton beams [1]. Currently, radiochomic films are used for online comparison with the simulated dose distribution after the irradiation but do not provide online verification. Hence, this study proposes to use the bremsstrahlung X-rays emitted by the medium as an in vivo dosimetry method. Previous studies have demonstrated the feasibility of using the bremsstrahlung yield to monitor the delivered dose at the entrance of a several-mm-thick PMMA phantom using a silicon drift detector SDD, with maximal intrinsic efficiency on the range 1-10 keV [2]. In addition, Ralite et al. have shown the yield's dependence on the beam energy and, therefore, the possibility of monitoring the incident beam energy. Material and Methods: In this work, an additional CdTe detector was used to detect X-ray energy up to 100 keV to access a broader spectrum, enabling a precise quantification of the main components of the bremsstrahlung, the Quasi Free Electron Bremsstrahlung (QFEB) (up to 37 keV for 70 MeV protons) and the Secondary Electron Bremsstrahlung (SEB) (mainly under 100 keV). Several calibrated tissue substitute cylinders (solid water, breast, lungs, bone - Gammex-RMI, WI, USA) were irradiated to investigate the spectrum dependencies on medium density and effective atomic number with proton energy. The information on dose and energy have been simulated with the code Monte Carlo GATE. The diameter of the cylinders is 28 mm, mimicking the mouse size, and the beam spot was 10 mm. The cylinders were positioned on a motorized translational axis in the beam propagation direction, and the detectors were shielded with lead to scan in depth by 5 mm slices. Results: Spectra at different depths and in various materials were acquired with both detectors. The method's sensitivity to dose was studied, and despite medium auto-attenuation, the bremsstrahlung X-rays were detected even in the Bragg peak region. Moreover, several characteristics of the bremsstrahlung spectrum (mean energy, FWHM, etc.) were studied to establish a direct link with the beam energy in the medium and its chemical composition. Conclusion and perspectives: This work is a comprehensive experimental study of bremsstrahlung spectra dependencies, for preclinical in vivo dosimetry verification. Analytical and Monte Carlo simulations will be used to optimize experimental set-up and to provide information on absorbed dose maps in mice, in relation to bremsstrahlung spectra, allowing the online comparison with treatment planning in a preclinical context. [1] M. Evin et al.,« Methodology for small animals targeted irradiations at conventional and ultra-high dose rates 65 MeV proton beam », Physica Medica, vol. 120, p. 103332, avr. 2024, doi: 10.1016/j.ejmp.2024.103332. [2] F. Ralite et al., « Bremsstrahlung X-rays as a non-invasive tool for ion beam monitoring », NIM-B, vol. 500-501, p. 76-82, août 2021, doi: 10.1016/j.nimb.2021.05.013.