Use of radiation protraction to escalate biologically effective dose to the treatment target

Purpose: The aim of this study is to evaluate how simultaneously increasing fraction time and dose per fraction affect biologically effective dose for the target (BED(tar)) while biologically effective dose for the normal tissue (BED(nt)) is fixed. Methods: In this investigation, BED(tar) and BED(nt) were studied by assuming mono-exponential repair of sublethal damage with tissue dependent repair half-time. Results: Our results demonstrate that under certain conditions simultaneously increasing fraction time and dose per fraction result in increased BEDtar while BEDnt is fixed. The dependence of biologically effective dose on fraction time is influenced by the dose rate. In this investigation we analytically determined time-varying dose rate (R) over tilde R which minimizes BED. Changes in BED with fraction time were compared for constant dose rate and for (R) over tilde. Conclusions: A number of recent experimental and theoretical studies have demonstrated that slow delivery of radiation (known as radiation protraction) leads to reduced therapeutic effect because of increased repair of sublethal damage. In contrast, our analysis shows that under certain conditions simultaneously increasing fraction time and dose per fraction are radiobiologically advantageous. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3656053]