One of the major limitations of radiotherapy is that the cells of solid tumors become deficient in oxygen. Solid tumors can outgrow their blood supply, causing a low-oxygen state known as hypoxia. Oxygen is a potent<unk> , increasing the effectiveness of a given dose of radiation by forming DNA-damaging free radicals. Tumor cells in a hypoxic environment may be as much as 2 to 3 times more resistant to radiation damage than those in a normal oxygen environment. Much research has been devoted to overcoming this problem including the use of high pressure oxygen tanks, blood substitutes that carry increased oxygen, hypoxic cell<unk> s such as misonidazole and metronidazole, and hypoxic cytotoxins, such as tirapazamine.
Radiosensitizer
Irradiation
Immunologic adjuvant
Immunostimulant
Mitogen
Sterilization 
Radiation
Neuroenhancement