The authors present the case of an 83-year-old man who underwent rectal resection due to cancer. In the further complementary treatment, radiotherapy and chemotherapy were applied. Following radiotherapy, the patient developed a rectal enterocutaneous fistula. Computed tomography revealed a small amount of fluid, the presence of pathological fluid collections located intermuscularly and interfascially, as well as a significant amount of gas bubbles and air spaces at the level of the left buttock. Below the level of the left trochanter was located massive emphysema of the soft tissues of the thigh, knee, lower leg, and partially of the foot. Despite treatment, the patient’s condition gradually deteriorated, ultimately leading to death. The authors conducted a review of the global literature and, in relation to the presented case, discuss the rare complication of radiotherapy: gas gangrene of the lower limbs.