The Response of Tissues to Very Low Doses per Fraction: A Reflection of Induced Repair?
The linear-quadratic (LQ) equation is now used widely to model and predict the increase in total dose with decreasing dose per fraction needed for an isoeffective response to radiotherapy in normal tissues and tumors (Joiner 1989). This relationship reflects the gradual decrease in radiation effectiveness with lowered doses due to these doses being further and further back “on the shoulder” of the underlying survival curve for the cells at risk. Although there has been some indication in experimental studies in vivo that the LQ model can underpredict radiation response to doses per fraction in the range 1–2 Gy relative to higher doses per fraction, this appears to be due to incomplete repair between successive doses (Thames et al. 1988). Alternatively, studies on pig skin have suggested that the LQ model can overpredict radiation effect in the range 2–6 Gy, but this has been attributed to a combination of cellular repopulation and cell cycle redistribution affecting net tissue response (Hopewell and van den Aardweg 1991). Thus for doses greater than 1 Gy per fraction, the LQ model describes the radiation response of normal tissues reasonably well.
KeywordsNeutron Dose Incomplete Repair Ionize Radiation Damage Cellular Repopulation Radiat Oncol Bioi Phys
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