title: Target volume geometric change and/or deviation from the cranium during fractionated stereotactic radiotherapy for brain metastases: potential pitfalls in image guidance based on bony anatomy alignment.
abstract: BACKGROUND
  This study sought to evaluate the potential geometrical change and/or displacement of the target relative to the cranium during fractionated stereotactic radiotherapy (FSRT) for treating newly developed brain metastases.
  
  
  METHODS
  For 16 patients with 21 lesions treated with image-guided frameless FSRT in 5 or 10 fractions using a 6-degree-of-freedom image guidance system-integrated platform, the unenhanced computed tomography or T2-weighted magnetic resonance images acquired until the completion of FSRT were fused to the planning image datasets for comparison. Significant change was defined as ≥3-mm change in the tumour diameter or displacement of the tumour centroid.
  
  
  RESULTS
  FSRT was started 1 day after planning image acquisition. Tumour shrinkage, deviation and both were observed in 2, 1 and 1 of the 21 lesions, respectively, over a period of 7-13 days. Tumour shrinkage or deviation resulted in an increase or decrease in the marginal dose to the tumour, respectively, and a substantial increase in the irradiated volume for the surrounding tissue irrespective of the pattern of alteration. No obvious differences in the clinical and treatment characteristics were noted among the populations with or without significant changes in tumour volume or position.
  
  
  CONCLUSIONS
  Target deformity and/or deviation can unexpectedly occur even during relatively short-course FSRT, inevitably leading to a gradual discrepancy between the planned and actually delivered doses to the tumour and surrounding tissue. To appropriately weigh the treatment outcome against the planned dose distribution, target deformity and/or deviation should also be considered in addition to the immobilisation accuracy, as image guidance with bony anatomy alignment does not necessarily guarantee accurate target localisation until completion of FSRT.
authors.affiliation: Department of Radiology, Gifu University Graduate School of Medicine, Gifu, Japan; Division of Radiation Oncology, Gifu University Hospital, Gifu, Japan.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Ohtakara
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Hoshi
mesh_terms: D000368:Aged; D001932:Brain Neoplasms / Q000175:diagnosis / Q000556:secondary* / Q000601:surgery*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D008297:Male; D011859:Radiography; D016634:Radiosurgery / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012886:Skull / Q000000981:diagnostic imaging* / Q000473:pathology*; D025321:Surgery, Computer-Assisted / Q000379:methods; D016896:Treatment Outcome; D047368:Tumor Burden
keywords: brain metastases; fractionated stereotactic radiotherapy; image guidance; internal change; treatment uncertainty