The Dark Side of Radiotherapy: Experimental Evidence, Biological Mechanisms, and Preventive Strategies for Radiation-Induced Tumor-Promoting Niches

Radiotherapy (RT) is an essential treatment modality for solid tumors and achieves tumor control primarily through radiation-induced cytotoxicity. However, its biological effects extend beyond direct tumor-cell killing. Radiation inevitably affects surrounding normal and tumor-adjacent tissues, inducing inflammatory, stromal, extracellular-matrix, vascular, and cellular remodeling that can persist after treatment. Accumulating experimental evidence indicates that these tissue responses can generate microenvironments permissive to the survival and regrowth of residual tumor cells, local reseeding, and metastatic colonization. This review examines radiation-induced tumor-promoting niches as an unintended consequence of tissue injury and repair. Experimental and clinical evidence supporting their existence is summarized, followed by discussion of the major biological processes underlying their formation, including immune remodeling, fibroblast activation and extracellular-matrix reorganization, vascular injury and hypoxia-driven reparative vasculogenesis, and cellular senescence. Emerging strategies to prevent or attenuate these niches are also discussed, including limiting normal-tissue radiation exposure and selectively targeting persistent inflammatory and stromal responses, post-radiation vasculogenesis, and senescent cells. Although direct causal evidence remains predominantly preclinical and the clinical significance of these responses remains to be established, understanding how irradiated tissues evolve after treatment may reveal opportunities to preserve the therapeutic benefits of RT while minimizing long-term tissue conditions that favor tumor recurrence or metastatic progression.