title: Intra- and interfractional variations for prone breast irradiation: an indication for image-guided radiotherapy.
abstract: OBJECTIVE
  Intra- and interfractional errors for breast cancer patients undergoing breast irradiation in the prone position were analyzed.
  
  
  METHODS
  To assess intrafractional error resulting from respiratory motion, four-dimensional computed tomography scans were acquired for 3 prone and 3 supine patients, and the respiratory motion was compared for the two positions. To assess the interfractional error caused by daily set-up variations, daily electronic portal images of one of the treatment beams were taken for 15 prone-positioned patients. Portal images were then overlaid with images from the planning system that included the breast contour and the isocenter, treatment beam portal, and isocenter. The shift between the planned and actual isocenter was recorded for each portal image, and descriptive statistics were collected for each patient. The margins were calculated using the 2Sigma + 0.7sigma recipe, as well as 95% confidence interval based on the pooled standard deviation of the datasets.
  
  
  RESULTS
  Respiratory motion of the chest wall is drastically reduced from 2.3 +/- 0.9 mm in supine position to -0.1 +/- 0.4 mm in prone position. The daily set-up errors vary in magnitude from 0.0 cm to 1.65 cm and are patient dependent. The margins were defined by considering only the standard deviation to be 1.1 cm, and 2.0 cm when the systematic errors were considered using the 2Sigma + 0.7sigma recipe.
  
  
  CONCLUSIONS
  Prone positioning of patients for breast irradiation significantly reduces the uncertainty introduced by intrafractional respiratory motion. The presence of large systematic error in the interfractional variations necessitates a large clinical target volume-to-planning target volume margin and indicates the importance of image guidance for partial breast irradiation in the prone position, particularly using imaging modality capable of identifying the lumpectomy cavity.
authors: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI 53226, USA.
authors: Natalya V
authors: NV
authors: Morrow
authors: Christopher
authors: C
authors: Stepaniak
authors: Julia
authors: J
authors: White
authors: J Frank
authors: JF
authors: Wilson
authors: X Allen
authors: XA
authors: Li
mesh_terms: D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D016001:Confidence Intervals; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000502:physiology; D009068:Movement; D016684:Prone Position; D012119:Respiration; D016683:Supine Position; D014057:Tomography, X-Ray Computed / Q000379:methods*