title: Investigation of the change in marker geometry during respiration motion: a preliminary study for dynamic-multi-leaf real-time tumor tracking.
abstract: BACKGROUND
  The use of stereotactic body radiotherapy (SBRT) is rapidly increasing. Presently, the most accurate method uses fiducial markers implanted near the tumor. A shortcoming of this method is that the beams turn off during the majority of the respiratory cycle, resulting in a prolonged treatment time. Recent advances in collimation technology have enabled continuous irradiation to a moving tumor. However, the lung is a dynamic organ characterized by inhalation exhalation cycles, during which marker/tumor geometry may change (i.e., misalignment), resulting in under-dosing to the tumor.
  
  
  RESULTS
  Eight patients with lung cancer who were candidates for stereotactic radiotherapy were examined with 4D high-resolution CT. As a marker surrogate, virtual bronchoscopy using the pulmonary artery (VBPA) was conducted. To detect possible marker/tumor misalignment during the respiration cycle, the distance between the peripheral bronchus, where a marker could be implanted, and the center of gravity of a tumor were calculated for each respiratory phase. When the respiration cycle was divided into 10 phases, the median value was significantly larger for the 30%-70% respiratory phases compared to that for the 10% respiratory phase (P<0.05, Mann-Whitney U-test).
  
  
  CONCLUSIONS
  These results demonstrate that physiological aspect must be considered when continuous tumor tracking is applied to a moving tumor. To minimize an "additional" internal target volume (ITV) margin, a marker should be placed approximately 2.5 cm from the tumor.
authors.affiliation: Graduate School of Health Sciences, Hokkaido University, Sapporo, Japan.
authors.forename: Rie
authors.initials: R
authors.lastname: Yamazaki
authors.forename: Seiko
authors.initials: S
authors.lastname: Nishioka
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Date
authors.forename: Hiroki
authors.initials: H
authors.lastname: Shirato
authors.forename: Takao
authors.initials: T
authors.lastname: Koike
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nishioka
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009038:Motion; D061330:Multidetector Computed Tomography; D016634:Radiosurgery / Q000379:methods*; D012119:Respiration; D047368:Tumor Burden