Oxygen is effective during radiotherapy?
Correct Answer: Just before staing
Description: Just before staing REF: Medical Radiology & Radiation Oncology by L. W. Brady, Philadelphia H.-P. Heilmann, Hamburg M. Molls, Munich page 223, 229, "The oxygenation just before radiotherapy is given to render tumor cells more radiosensitive, and hypoxia is created during the radiotherapy to protect normal tissue from radiotherapy" For complete understanding read following text: The use of oxygen in radiotherapy is based on the knowledge of the way in which the radio-sensitivity of cells varies with the oxygen tension in their immediate environment at the time of irradiation and on the belief that some tumours contain foci of anoxic cells (Gray et al., 1953; Thomlinson and Gray, 1955; Gray, 1957). Gray and his associates made the following observation in 1953: "The radiation effect on living cells in anoxia rapidly increases with oxygenation until it reaches a near maximum effect at the P02 found in normal capillaries and tissue fluids-40 mm. Hg. Fuher increase in oxygenation does not appreciably increase the radiation effect. Therefore, unlike most cancerocidal drugs used to potentiate radiation effect, oxygen does not increase the response in normal tissues. It thus constitutes the ideal adjuvant." There are four "R"s of radiotherapy (Withers 1992; Thames et al. 1989) and include repair of sublethal cell damage, repopulation of tumour cells, redistribution of cells over different cell cycle phases with different radio-sensitivity and reoxygenation of radio-resistant hypoxic tumours after beginning of the radiotherapy course. Repair and repopulation have been identifi ed as the most impoant factors with respect to radiation response and were therefore subject of early models. A special situation arises for hypoxic tumours, in which the radio-sensitivity of cells is significantly reduced. This effect may in principal be described by introducing the oxygen enhancement ratio defined by: OER = D hypox/D Where D and Dhypox are biologically iso-effective doses for well-oxygenated and hypoxic tumours. Typical values of the OER vary between 1 (well oxygenated) and 3 (strongly hypoxic).
Category:
Radiology
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