title: Theoretical implications of incorporating relative biological effectiveness into radiobiological equivalence relationships.
abstract: OBJECTIVE
  Earlier radiobiological equivalence relationships as derived for low-linear energy transfer (LET) radiations are revisited in the light of newer radiobiological models that incorporate an allowance for relative biological effectiveness (RBE).
  
  
  METHODS
  Linear-quadratic (LQ) radiobiological equations for calculating biologically effective dose at both low- and high-LET radiations are used to derive new conditions of equivalence between a variety of radiation delivery techniques. The theoretical implications are discussed.
  
  
  RESULTS
  The original (pre-LQ) concept of equivalence between fractionated and continuous radiotherapy schedules, in which the same physical dose is delivered in each schedule, inherently assumed that low-LET radiation would be used in both schedules. LQ-based equivalence relationships that allow for RBE and are derived assuming equal total physical dose between schedules are shown to be valid only in limited circumstances. Removing the constraint of equality of total physical dose allows the identification of more general (and more practical) relationships.
  
  
  CONCLUSIONS
  If the respective schedules under consideration for equivalence both involve radiations of identical LET, then the original equivalence relationships remain valid. However, if the compared schedules involve radiations of differing LET, then new (and more restrictive) equivalence relationships are found to apply.
  
  
  CONCLUSIONS
  Theoretically derived equivalence relationships based on the LQ model provide a framework for the design and intercomparison of a wide range of clinical techniques including those involving high- and/or low-LET radiations. They also provide a means of testing for the validity of variously assumed tissue repair kinetics.
authors: Particle Therapy Cancer Research Institute, University of Oxford, Oxford, UK. richard.holloway@ptcri.ox.ac.uk
authors: R P
authors: RP
authors: Holloway
authors: R G
authors: RG
authors: Dale
mesh_terms: D001918:Brachytherapy / Q000592:standards*; D002470:Cell Survival / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D018499:Linear Energy Transfer / Q000528:radiation effects; D008954:Models, Biological; D011853:Radiobiology / Q000592:standards*; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness
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