The main challenge of radiotherapy is to maximize the dose of radiation to tumor cells while minimizing damage to the surrounding healthy tissue.[1] The probability of curing the tumor increases with increasing total radiation dose, but the risk of severe late damage to normal tissue also increases.[1] Normal tissue responses to radiation have a threshold dose below which no response occurs and saturate at high doses; the relationship is steep, so small changes in dose lead to large differences in toxicity.[1] A 20% dose increase in radiation-resistant patients can increase tumor control by 20–40%, improving survival.[1] Conversely, reducing the dose by 20% in susceptible patients would almost eliminate severe side effects.[1] Patients with extreme radiosensitivity would benefit from genetic profiling, with 50% of patients benefiting: 40% increased tumor control and 10% decreased morbidity.[1] Symptoms of normal tissue damage may appear during therapy, weeks after, or months to years later as a result of cell death or wound healing.[2] Treatments are being developed to reduce the risk or severity of damage and promote healing.[2]