title: Correlation of lung tumor motion with external surrogate indicators of respiration.
abstract: OBJECTIVE
  To assess the correlation of respiratory volume and abdominal displacement with tumor motion as seen with X-ray fluoroscopy. Measurements throughout the patient's treatment course allowed an assessment of the interfractional reproducibility of this correlation.
  
  
  METHODS
  Data were acquired from 11 patients; 5 were studied over multiple days. Measurements of respiratory volume by spirometry and abdominal displacement by a real-time position tracking system were correlated to simultaneously acquired X-ray fluoroscopy measurements of superior-inferior tumor displacement. The linear correlation coefficient was computed for each data acquisition. The phase relationship between the surrogate and tumor signals was estimated through cross-correlation delay analysis.
  
  
  RESULTS
  Correlation coefficients ranged from very high to very low (0.99-0.39, p < 0.0001). The correlation between tumor displacement and respiratory volume was higher and more reproducible from day to day than between tumor displacement and abdominal displacement. A nonzero phase relationship was observed in nearly all patients (-0.65 to +0.50 s). This relationship was observed to vary over inter- and intrafractional time scales. Only 1 of 5 patients studied over multiple days had a consistent relationship between tumor motion and either surrogate.
  
  
  CONCLUSIONS
  Respiratory volume has a more reproducible correlation with tumor motion than does abdominal displacement. If forming a tumor-surrogate prediction model from a limited series of observations, the use of surrogates to guide treatment might result in geographic miss.
authors.affiliation: Department ofMedical Biophysics, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Jeremy D P
authors.initials: JD
authors.lastname: Hoisak
authors.forename: Katharina E
authors.initials: KE
authors.lastname: Sixel
authors.forename: Romeo
authors.initials: R
authors.lastname: Tirona
authors.forename: Patrick C F
authors.initials: PC
authors.lastname: Cheung
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Pignol
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D005471:Fluoroscopy; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D008297:Male; D008875:Middle Aged; D009068:Movement; D015203:Reproducibility of Results; D012119:Respiration; D013147:Spirometry