title: Normal tissue dose conformality measures to guide radiotherapy fractionation decisions.
abstract: OBJECTIVE
  To determine conditions under which hypofractionation could be favorable for a normal tissue--even if tumor [alpha/beta] exceeds the normal tissue's [alpha/beta].
  
  
  METHODS
  The hypofractionation sufficiency condition (HSC) for an organ is defined as a dose conformality constraint such that, if satisfied, a family of tumor control probability isoeffective fractionation schemes will show decreasing normal tissue complication probability with decreasing number of fractions.
  
  
  RESULTS
  In the extended equivalent uniform dose (EUD) model [obtained by replacing dose with linear quadratic (LQ) 2 Gy equivalent dose], the HSC for a normal organ is proven to be satisfied if a suitably weighted average of the relative dose [hypofractionation sufficiency index (HSI)] is less than the ratio of normal tissue to tumor [alpha/beta]. The HSI is determined solely by dose distribution and the normal tissue volume factor, "a." If the HSC is satisfied for every normal tissue of concern, then there is a therapeutic gain with hypofractionation. The corresponding multifractionation sufficiency condition (therapeutic gain with increasing number of fractions) and multifractionation sufficiency index (MSI) are also derived. A sample clinical case is presented.
  
  
  CONCLUSIONS
  Within the context of the LQ/EUD models, conformality measures (HSI and MSI) can be used to inform fractionation decisions.
authors.affiliation: Department of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA. myerson@radonc.wustl.edu
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Myerson
mesh_terms: D019583:Dose Fractionation, Radiation; D006801:Humans; D008113:Liver Neoplasms / Q000532:radiotherapy / Q000556:secondary; D058958:Organs at Risk / Q000528:radiation effects; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*