title: Differences in brainstem fiber tract response to radiation: a longitudinal diffusion tensor imaging study.
abstract: OBJECTIVE
  To determine whether radiation-induced changes in white matter tracts are uniform across the brainstem.
  
  
  METHODS
  We analyzed serial diffusion tensor imaging data, acquired before radiation therapy and over 48 to 72 months of follow-up, from 42 pediatric patients (age 6-20 years) with medulloblastoma. FSL software (FMRIB, Oxford, UK) was used to calculate fractional anisotropy (FA) and axial, radial, and mean diffusivities. For a consistent identification of volumes of interest (VOIs), the parametric maps of each patient were transformed to a standard brain space (MNI152), on which we identified VOIs including corticospinal tract (CST), medial lemniscus (ML), transverse pontine fiber (TPF), and middle cerebellar peduncle (MCP) at the level of pons. Temporal changes of DTI parameters in VOIs were compared using a linear mixed effect model.
  
  
  RESULTS
  Radiation-induced white matter injury was marked by a decline in FA after treatment. The decline was often accompanied by decreased axial diffusivity, increased radial diffusivity, or both. This implied axonal damage and demyelination. We observed that the magnitude of the changes was not always uniform across substructures of the brainstem. Specifically, the changes in DTI parameters for TPF were more pronounced than in other regions (P<.001 for FA) despite similarities in the distribution of dose. We did not find a significant difference among CST, ML, and MCP in these patients (P>.093 for all parameters).
  
  
  CONCLUSIONS
  Changes in the structural integrity of white matter tracts, assessed by DTI, were not uniform across the brainstem after radiation therapy. These results support a role for tract-based assessment in radiation treatment planning and determination of brainstem tolerance.
authors.affiliation: Department of Radiological Sciences, St. Jude Children's Research Hospital, Memphis, Tennessee, USA. jinsoo.uh@stjude.org
authors.forename: Jinsoo
authors.initials: J
authors.lastname: Uh
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Merchant
authors.forename: Yimei
authors.initials: Y
authors.lastname: Li
authors.forename: Tianshu
authors.initials: T
authors.lastname: Feng
authors.forename: Amar
authors.initials: A
authors.lastname: Gajjar
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Ogg
authors.forename: Chiaho
authors.initials: C
authors.lastname: Hua
mesh_terms: D000293:Adolescent; D016880:Anisotropy; D001933:Brain Stem / Q000528:radiation effects*; D002528:Cerebellar Neoplasms / Q000532:radiotherapy*; D002648:Child; D056324:Diffusion Tensor Imaging / Q000379:methods; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008527:Medulloblastoma / Q000532:radiotherapy*; D011149:Pons / Q000528:radiation effects; D011446:Prospective Studies; D011712:Pyramidal Tracts / Q000528:radiation effects; D011832:Radiation Injuries / Q000175:diagnosis*; D055815:Young Adult