BACKGROUND Anal squamous cell carcinoma (SCC) is a rare cancer commonly treated with the Nigro protocol, which combines chemotherapy and radiation. Patients who received radiation therapy prior to modern advances, such as computer-based tumor targeting, volumetric planning, and intensity-modulated radiation therapy, experience more acute and chronic adverse effects. Though exceedingly rare, radiation necrosis is of particular concern, as it can result in significant morbidity and mortality, including complex pelvic fistula formation and predisposition to potentially life-threatening necrotizing soft-tissue infections. CASE REPORT Here, we present a case of a 66-year-old woman with a prior history of anal SCC stage T3N×M0 who was treated with the Nigro protocol. Her treatment course was complicated by radiation proctitis, necessitating fecal diversion and ureteral strictures, requiring frequent stent exchanges. She presented 18 years after her cancer treatment, with widespread necrosis of her pelvic organs and surrounding soft tissue, resulting in formation of a large pelvic "cloaca", with a superimposed necrotizing soft-tissue infection. She was successfully treated by expedited resuscitation, septic source control, using multiple extensive debridements, and complete urinary diversion, utilizing a multidisciplinary team. CONCLUSIONS This case highlights the importance of monitoring patients for signs of radiation toxicity, particularly in patients who received radiation prior to the latest technological advancements, as they are at increased risk of developing severe, late adverse effects decades after treatment. When these complications are recognized, early and aggressive intervention is required to spare the patient significant morbidity and mortality.