Introduction Cells possess sophisticated defenses that preserve their genetic material, regulate division, and respond to DNA damage. DNA damage response proteins and tumor suppressor genes function as genome guardians, preventing genetic errors from being passed to subsequent generations of cells. However, these defenses can fail when damage is complex or repaired incorrectly. Ionizing radiation can damage DNA. An affected cell may repair the damage, die if the injury is severe, or survive with genetic alterations that could contribute to cancer development. “Cellular betrayal” therefore serves as a metaphor for disrupted protective mechanisms, rather than an inevitable consequence of radiation exposure. NCI How Radiation Damages Genetic Material Ionizing radiation can damage DNA directly or act indirectly through water radiolysis, which generates reactive chemical species capable of attacking cellular components. The resulting lesions include damaged bases, single-strand breaks, and double-strand breaks. Double-strand breaks are particularly hazardous because repair must reconnect the correct DNA segments while preserving genetic organization. Closely spaced, complex lesions make accurate restoration more difficult. CDC When Repair Becomes a Source of Error Cells repair double-strand breaks through several pathways. Homologous recombination (HR) generally uses a matching DNA template, whereas nonhomologous end joining (NHEJ) reconnects broken ends without requiring an extensive homologous template. End joining is essential for maintaining chromosome integrity. Nevertheless, repair can alter the DNA sequence when damaged ends require processing. If several breaks occur, ends from different genomic locations may be joined, generating chromosomal translocations or other rearrangements. Experiments in human cells have shown that targeted breaks on different chromosomes can produce translocations, illustrating why correct end selection matters during repair. NCBI Bookshelf Genome Guardians: Loss of Protection and Harmful New Functions The p53 protein, encoded by TP53, is a prominent genome guardian. It helps coordinate cellular responses to damage and can promote cell-cycle arrest, senescence, or programmed cell death when cellular integrity cannot be maintained. Some mutations impair its tumor-suppressive activity. Certain mutant forms can also acquire additional functions that support tumor behavior. Animal studies have demonstrated that the effects of particular p53 mutations can extend beyond the simple absence of normal p53. These effects depend on the specific mutation and cellular context and do not apply to every mutant form. pubmed.ncbi.nlm.nih.gov Thus, a guardian may contribute to tumor development through loss of protection or, in specific circumstances, acquisition of harmful functions. This does not mean that radiation directly converts every protective protein into a cancer-promoting protein, or that every TP53 mutation results from radiation exposure. Genomic Evidence from Radiation-Associated Tumors A study published in Nature Communications in 2016 investigated the complete genomes of twelve second malignancies associated with previous radiotherapy. Compared with tumors without previous therapeutic radiation exposure, these cancers showed an excess of small deletions and balanced chromosomal inversions. These findings support a link between radiation damage and structural genomic alterations capable of affecting cancer-related genes. However, the investigators also emphasized the uncertainty involved in attributing an individual tumor to radiation. Genomic findings must be interpreted alongside clinical and research evidence. Nature Communications Relevance to Medical Physics Understanding these mechanisms explains the importance of justifying medical exposures, optimizing doses, and protecting healthy tissues. Radiotherapy deliberately exploits DNA damage to eliminate cancer cells while limiting irradiation of surrounding tissues. The possibility of second malignancies does not negate the therapeutic value of radiation. Long-term risks are weighed against the expected benefits of treatment. Similarly, medically justified imaging procedures provide diagnostic benefits that generally outweigh their small radiation-related risks. NCI Conclusion “Cellular betrayal” highlights that survival after DNA damage does not necessarily mean restoration of genetic integrity. Incorrect repair may allow a cell to persist with alterations that weaken growth control or disable tumor suppressor genes. Nevertheless, cancer develops through a multistep process, and neither DNA damage nor a repair error inevitably produces a tumor. Understanding this relationship connects radiation biology with medical physics and supports the effective, safe use of radiation. Keywords: Ionizing radiation, DNA repair, genomic instability, p53, tumor suppressor genes, radiation carcinogenesis. Almustaqbal University – The First University in Iraq