"Radiation must generally produce double-stranded breaks in DNA to kill a cell, owing partly to the high capacity of mammalian cells for repairing single-strand damage. Radiation can also produce effects indirectly by interacting with water (which makes up approximately 80 percent of a cell's volume) to generate free radicals, which can damage the cell. Free radicals are highly reactive chemical entities that lack a stable number of outer-shell electrons. A free radical is not stable and has a life span of a fraction of a second. It is estimated that most x-ray--induced cell damage is due to the formation of hydroxyl radicals..." [Harrison's Principles of Internal Medicine, 14th Edition, page 2560.]

What makes a tumor radiosensitive? "It is known that radiation therapy can be successfully used to cure or control some types of human tumors, while consistently failing in others. This has been ascribed to several factors including differences in the intrinsic sensitivity of the tumor cells and in their ability to recover from radiation damage. In this study, human tumor cells from an osteogenic sarcoma, a glioblastoma, and two medulloblastomas, as well as cells from human skin, were established in tissue culture, and ...survival ... determined. No significant differences in ... survival ... could be detected among these human tumors or skin cells, despite the wide variability in their radiocurability. In addition, skin cell strains derived from patients exhibiting markedly sensitive or resistant skin reactions during fractionated radiotherapy showed no differences in survival characteristics from normal controls. <b>It is therefore suggested that the wide range of radiocurabilities seen among various human tumors cannot be explained on the basis of inherent cellular factors responsible for the survival of tumor cells after x-irradiation."</b> 
J Nucl Med 1998 Sep;39(9):1551-4 
&&url PMID: 1069484

Tumor damage depends upon amount of hypoxic cells [which are radioresistant], radiation dose, doses of chemotherapy and/or other radiosensitizer given, ambient temperature, timing of drug dose and radiation exposures. The effectiveness of oxygenating the hypoxic cells to make them radiosensitive depends upon how densely the tissue is vascularized, hemoglobin concentrations and affinities for oxygen, and levels of carbon dioxide breathed in [causes blood vessels to dilate and deliver more blood to tissues], as well as use of hyperbaric [high pressure] oxygen treatment.

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