Cancer treatments by photon radiation beams in radiotherapy is currently grounded on a rather empirical understanding of the basic mechanisms of DNA damages. It is well assessed that radiation induces single-strand and double-strand breaks in DNA, being however the various mechanisms largely unknown. Conversely, the mechanical behaviour of undamaged DNA is today well understood, both experimentally and theoretically [1]. Thanks to an ad-hoc developed Silicon Nanotweezer (SNT) [2], we can perform direct measurements of the mechanical response of DNA bundles under irradiation. This allows to measure, with high precision, the rate of breaking of DNA as a function of the various external agents. Grounded in our previous developments in polymer physics [3, 4], a theoretical investigation of DNA damage has been introduced to complement experiments. We formulate analytical models and perform Monte Carlo simulations, to quantitatively evaluate the visco-elastic response for DNA bundles, which experience breaks induced by a photon radiation beam. The numerical simulations confirm the analytical achievements and allow to study situations closely representing the experimental conditions. [1] C. R. Calladine et al., Understanding DNA: The Molecule and How It Works (Elsevier Academic Press, Amsterdam, 2004). [2] C. Yamahata et al., JMEMS 17, no. 3 (2008) 623. [3] F. Manca et al,. J. Chem. Phys. 136 (2012) 154906. [4] F. Manca et al., Phys. Rev. E 87 (2013) 032705.