Disentangling Dose and Radiation Quality Effects on Late DNA Damage Response in Human Mesenchymal Stromal Cells

Although ionizing radiation quality shapes the spatial structure and complexity of primary DNA lesions, its contribution relative to absorbed dose in the late DNA damage response (DDR) remains poorly quantified. We analyzed γH2AX and pATM foci kinetics in human mesenchymal stromal/stem cells exposed to 60Co γ-rays, 150 MeV protons, and 14.1 MeV neutrons at doses of 0.25, 0.5, 1.0, and 2.0 Gy, delivered at dose rates of 0.5 Gy/min for γ-rays, 1 Gy/min for protons, and 46 mGy/min for neutrons, respectively. An effective kinetic model was used to describe the time course of the DDR and incorporated an ultraslow component as a model-based index of long-lasting repair burden. Radiation types differed less in early signal amplitude than in decay rate and late persistence. At every investigated dose, the normalized ultraslow fraction followed the order γ-rays < protons < neutrons. This ordering was preserved in per-dose fits, while parametric bootstrapping supported the γ–neutron separation, although the absolute values of the internal fractions were only partially identifiable. RAD51, analyzed independently from model fitting, showed the same qualitative ordering at late time points, supporting the interpretation that radiation quality is more associated with persistent DDR signaling than absorbed dose within the investigated range. These findings suggest that absorbed dose alone may not fully characterize the late cellular response to different radiation qualities and highlight the importance of radiation quality in comparative radiobiology, radiation protection, and the interpretation of normal-tissue responses to therapeutic irradiation.