title: Mitigating errors in external respiratory surrogate-based models of tumor position.
abstract: OBJECTIVE
  To investigate the effect of tumor site, measurement precision, tumor-surrogate correlation, training data selection, model design, and interpatient and interfraction variations on the accuracy of external marker-based models of tumor position.
  
  
  METHODS
  Cyberknife Synchrony system log files comprising synchronously acquired positions of external markers and the tumor from 167 treatment fractions were analyzed. The accuracy of Synchrony, ordinary-least-squares regression, and partial-least-squares regression models for predicting the tumor position from the external markers was evaluated. The quantity and timing of the data used to build the predictive model were varied. The effects of tumor-surrogate correlation and the precision in both the tumor and the external surrogate position measurements were explored by adding noise to the data.
  
  
  RESULTS
  The tumor position prediction errors increased during the duration of a fraction. Increasing the training data quantities did not always lead to more accurate models. Adding uncorrelated noise to the external marker-based inputs degraded the tumor-surrogate correlation models by 16% for partial-least-squares and 57% for ordinary-least-squares. External marker and tumor position measurement errors led to tumor position prediction changes 0.3-3.6 times the magnitude of the measurement errors, varying widely with model algorithm. The tumor position prediction errors were significantly associated with the patient index but not with the fraction index or tumor site. Partial-least-squares was as accurate as Synchrony and more accurate than ordinary-least-squares.
  
  
  CONCLUSIONS
  The accuracy of surrogate-based inferential models of tumor position was affected by all the investigated factors, except for the tumor site and fraction index.
authors.affiliation: Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Kathleen T
authors.initials: KT
authors.lastname: Malinowski
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: McAvoy
authors.forename: Rohini
authors.initials: R
authors.lastname: George
authors.forename: Sonja
authors.initials: S
authors.lastname: Dieterich
authors.forename: Warren D
authors.initials: WD
authors.lastname: D'Souza
mesh_terms: D000465:Algorithms; D057918:Fiducial Markers; D006801:Humans; D016018:Least-Squares Analysis; D008113:Liver Neoplasms; D008175:Lung Neoplasms; D008954:Models, Biological; D009068:Movement; D010190:Pancreatic Neoplasms; D016634:Radiosurgery / Q000379:methods; D012044:Regression Analysis; D012119:Respiration; D012189:Retrospective Studies; D013238:Stereotaxic Techniques