Surgery and radiation are hallmark oncology treatments routinely employed against a variety of cancers. Cancer cells that successfully evade these therapies often fuel and propagate fatal tumors. Uncovering fundamental mechanisms that allow these cells to escape is key to identifying therapies that will help target and permanently obliterate cancer cells. We have discovered two independent ways in which stem cells in planarians, flatworms that are inherently resistant to cancer, can survive radiation. Planarians have a large population of stem cells that rapidly proliferate and are essential for regeneration. These cells are also ordinarily radio-sensitive, and radiation-induced DNA damage results in rapid apoptosis, leaving animals with few to no surviving stem cells. We discovered that injury within a defined window of radiation resulted in stem cells that would otherwise die persisting locally around the wound-site. While diverse injuries were capable of driving stem cells to persist, the number of persisting cells was governed by the severity of the wound, with larger wounds retaining more cells. By combining fluorescence-activated cell sorting (FACS) with annexin V staining we found that stem cells were preserved on account of significantly delaying radiation-induced apoptosis. Additionally, stem cell persistence relied on the mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase (ERK), which is the earliest signal initiating regeneration and stem cell differentiation in planarians. Pharmacologically inhibiting ERK completely blocked stem cell retention after injury, implicating wound-induced ERK activity in this response. Intriguingly, despite stem cells surviving locally in large numbers, radiated injured animals failed to regenerate and ultimately died. While exploring additional pathways that might contribute to stem cell persistence, we unexpectedly discovered a crucial role for the highly conserved DNA damage response gene ATM. RNAi of atm resulted in stem cells persisting in astounding numbers throughout the body of the animal. This global stem cell survival occurred independent of injury, and was mediated by changes in the cell cycle. By strategically employing the thymidine analog EdU, weI found that persisting stem cells in radiated atm(RNAi) animals surpassed the G1/S checkpoint activated by radiation to accumulate in the S and G2 phase of the cell cycle. This checkpoint evasion allowed stem cells to potentially employ repair mechanisms, and they eventually resumed cycling. Unlike locally persisting stem cells, globally persisting stem cells were surprisingly competent, capable of fuelling both regeneration and long-term survival after radiation. Knockdown of additional DNA damage and cell cycle checkpoint genes identified via RNAseq of stem cells isolated from atm(RNAi) animals failed to independently drive stem cell survival.These results demonstrate that ATM primarily regulates radiation-induced apoptosis via activation of the G1/S checkpoint in planarians. This study establishes two unique, diametric paradigms in which stem cells harness highly conserved pathways to circumvent the effects of radiation. Impressively, despite the massive amounts of DNA-damage inflicted, we failed to see fateful consequences such as tumor formation in planarians. Future studies will focus on how planarian stem cells tightly regulate their fate in the face of genomic insults like DNA damage, with the hope of providing a novel lens that might help identify potential hidden targets.