<p>Radiation-induced lung injury (RILI) not only limits the therapeutic dose that can be administered during chest and thorax radiotherapy (RT), but also significantly impairs patients’ health and quality of life. RT-induced senescence and the associated altered secretory profile, the senescence-associated secretory phenotype (SASP), have emerged as a central process for the development and progression of pneumonitis and pulmonary fibrosis. Among various lung cell types, the phenomenon of permanent cell cycle arrest, which is also accompanied by significant morphological changes, has been observed especially in epithelial cells. RILI arises from a complex interplay of cell types and signaling pathways, but it has not yet been clarified when which lung cell types become senescent during RT and how induced changes in one cell type may influence senescence or the RT-dependent cell fate overall in another, adjacent cell type. Here, the different cellular fates particularly senescence versus apoptotic cell death following RT-induced genotoxic stress were investigated particularly in epithelial cells and fibroblasts providing further insights into the radiosensitivity of these lung cells. Fibroblasts that have become senescent during RT alter the RT response and thus the cell fate of co-cultured epithelial cells. In addition, new candidate genes were identified that were induced in the various cellular subpopulations of complex epithelial-fibroblast spheroids as an approximate in vivo cell culture model after irradiation. These candidate genes could be used in future studies as additional RT-induced gene sets and, in particular, as senescence-associated gene sets. A comprehensive understanding of the dynamic changes of these cellular components is crucial to specify new strategies for the prevention of RILI.</p>