title: Brain necrosis after fractionated radiation therapy: is the halftime for repair longer than we thought?
abstract: OBJECTIVE
  To derive a radiobiological model that enables the estimation of brain necrosis and spinal cord myelopathy rates for a variety of fractionation schemes, and to compare repair effects between brain and spinal cord.
  
  
  METHODS
  Sigmoidal dose response relationships for brain radiation necrosis and spinal cord myelopathy are derived from clinical data using nonlinear regression. Three different repair models are considered and the repair halftimes are included as regression parameters.
  
  
  RESULTS
  For radiation necrosis, a repair halftime of 38.1 (range 6.9-76) h is found with monoexponential repair, while for spinal cord myelopathy, a repair halftime of 4.1 (range 0-8) h is found. The best-fit alpha beta ratio is 0.96 (range 0.24-1.73).
  
  
  CONCLUSIONS
  A radiobiological model that includes repair corrections can describe the clinical data for a variety of fraction sizes, fractionation schedules, and total doses. Modeling suggests a relatively long repair halftime for brain necrosis. This study suggests that the repair halftime for late radiation effects in the brain may be longer than is currently thought. If confirmed in future studies, this may lead to a re-evaluation of radiation fractionation schedules for some CNS diseases, particularly for those diseases where fractionated stereotactic radiation therapy is used.
authors.affiliation: Department of Human Oncology, University of Wisconsin-Madison, School of Medicine and Public Health, Madison, WI 53792, USA. bender@humonc.wisc.edu
authors.forename: Edward T
authors.initials: ET
authors.lastname: Bender
mesh_terms: D001921:Brain / Q000473:pathology* / Q000528:radiation effects*; D019583:Dose Fractionation, Radiation; D006801:Humans; D008954:Models, Biological; D009336:Necrosis / Q000209:etiology; D011832:Radiation Injuries / Q000209:etiology*; D011853:Radiobiology; D013118:Spinal Cord Diseases / Q000209:etiology*; D013997:Time Factors