title: Regional normal lung tissue density changes in patients treated with stereotactic body radiation therapy for lung tumors.
abstract: OBJECTIVE
  To describe regional lung tissue density changes in normal lung tissue of patients with primary and metastatic lung tumors who received stereotactic body radiation therapy (SBRT).
  
  
  METHODS
  A total of 179 post-SBRT follow-up computed tomography (CT) scans of 62 patients who received SBRT between 2003 and 2009 were studied. Median prescription dose was 54 Gy (range, 30-60 Gy) in 3 to 5 fractions. SBRT-induced lung density changes on post-SBRT follow-up CT were evaluated at approximately 3, 6, 12, 18, 24, and 30 months after treatment. Dose-response curves (DRC) were generated for SBRT-induced lung damage by averaging CT number (HU) changes for regions of the lungs receiving the same dose at 5-Gy intervals.
  
  
  RESULTS
  For all follow-up interval periods, CT numbers linearly increased with dose until 35 Gy and were constant thereafter. For 3, 18, 24, and 30 months, the rate of relative electron density increase with dose was approximately 0.24% per Gy. At 6 months, the rate was also similar below 20 Gy but then rose to 0.6% per Gy above this threshold. After 6 months, DRCs were mostly time-independent. When split between patients treated with 3 fractions of 12 to 20 Gy (median, 20 Gy; average tumor volume, 12±16 cm3) and with >3 fractions of 6 to 12.5 Gy (median, 9 Gy; average tumor volume, 30±40 cm3), DRCs differed significantly. In both cases, CT changes at 3, 18, 24, and 30 months were identical to those of the population DRC; however, patients who received >3 fractions showed 6-month CT changes that were more than twice those for the group that received 3 fractions.
  
  
  CONCLUSIONS
  This analysis of SBRT-induced normal lung density changes indicates that lung normal tissue has more pronounced self-limited acute effects than late effects. Differences in acute CT changes following treatments in 3 fractions were considerably less than for treatments in >3 fractions.
authors.affiliation: Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, Colorado 80045, USA. quentin.diot@ucdenver.edu
authors.forename: Quentin
authors.initials: Q
authors.lastname: Diot
authors.forename: Brian
authors.initials: B
authors.lastname: Kavanagh
authors.forename: Tracey
authors.initials: T
authors.lastname: Schefter
authors.forename: Laurie
authors.initials: L
authors.lastname: Gaspar
authors.forename: Kelly
authors.initials: K
authors.lastname: Stuhr
authors.forename: Moyed
authors.initials: M
authors.lastname: Miften
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000528:radiation effects*; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000556:secondary / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011832:Radiation Injuries / Q000000981:diagnostic imaging; D011859:Radiography; D016634:Radiosurgery / Q000009:adverse effects* / Q000379:methods; D012189:Retrospective Studies; D047368:Tumor Burden