Effect of dose rate in hypofractionated radiotherapy

Published:September 06, 2019DOI:


      • Effect of dose rate on biologically effective dose in hypofractionated regimens can be significant (i.e., >5%).
      • In the case of low dose rate, hypofractionation can lead to a significantly increased BED in the organ at risk.
      • Current study indicates that hypofractionated treatments should be delivered with the highest dose rate available.



      To evaluate how dose rate affects radiobiological properties of hypofractionated radiotherapy.


      This study is based on the linear-quadratic (LQ) model used to determine biologically effective dose ( BED ). Changes in the biologically effective dose in normal tissue ( BE D nt ) are studied as a function of number of fractions and dose rate under the condition of fixed BED in the treatment target ( BE D tar ) .


      In this study we demonstrate that compared to standard fractionation, hypofractionation can either decrease or increase BE D nt depending on the average dose rate. In the considered examples, maximum value of BE D nt in the spinal cord varies monotonically with number of fractions ( N f ) when dose rate is sufficiently high so that the corresponding fraction time is much smaller than characteristic repair half-lives for malignant and normal cells. In contrast, in the case of a lower dose rate of 300 MU/min, BE D nt in the cord can vary non-monotonically with N f . In the later case, there exists optimum number of fractions which corresponds to the minimum BE D nt . It is shown that in the case when radiation induced sublethal damage is repaired faster in the target than in the affected organ at risk (OAR), increasing dose rate helps lower BE D nt .


      We have demonstrated that, as compared to standard fractionation, hypofractionation can either increase or decrease BE D nt in the OAR depending on the utilized dose rate. Consequently, radiobiological assessment of hypofractionation should take into account dose rate as well as repair rates in the target and OAR.


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