title: Estimation of optimal matching position for orthogonal kV setup images and minimal setup margins in radiotherapy of whole breast and lymph node areas.
abstract: OBJECTIVE
  The aim was to find an optimal setup image matching position and minimal setup margins to maximally spare the organs at risk in breast radiotherapy.
  
  
  BACKGROUND
  Radiotherapy of breast cancer is a routine task but has many challenges. We investigated residual position errors in whole breast radiotherapy when orthogonal setup images were matched to different bony landmarks.
  
  
  METHODS
  A total of 1111 orthogonal setup image pairs and tangential field images were analyzed retrospectively for 50 consecutive patients. Residual errors in the treatment field images were determined by matching the orthogonal setup images to the vertebrae, sternum, ribs and their compromises. The most important region was the chest wall as it is crucial for the dose delivered to the heart and the ipsilateral lung. Inter-observer variation in online image matching was investigated.
  
  
  RESULTS
  The best general image matching position was the compromise of the vertebrae, ribs and sternum, while the worst position was the vertebrae alone (p ≤ 0.03). The setup margins required for the chest wall varied from 4.3 mm to 5.5 mm in the lung direction while in the superior-inferior (SI) direction the margins varied from 5.1 mm to 7.6 mm. The inter-observer variation increased the minimal margins by approximately 1 mm. The margin of the lymph node areas should be at least 4.8 mm.
  
  
  CONCLUSIONS
  Setup margins can be reduced by proper selection of a matching position for the orthogonal setup images. To retain the minimal margins sufficient, systematic error of the chest wall should not exceed 4 mm in the tangential field image.
authors.affiliation: Department of Oncology, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Marko
authors.initials: M
authors.lastname: Laaksomaa
authors.affiliation: Department of Oncology, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland ; Department of Medical Physics, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Mika
authors.initials: M
authors.lastname: Kapanen
authors.affiliation: Department of Oncology, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Tanja
authors.initials: T
authors.lastname: Skyttä
authors.affiliation: Department of Medical Physics, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Seppo
authors.initials: S
authors.lastname: Peltola
authors.affiliation: Department of Medical Physics, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Simo
authors.initials: S
authors.lastname: Hyödynmaa
authors.affiliation: Department of Oncology, Tampere University Hospital (Tays), PO Box 2000 (Teiskontie 35), FI-33521 Tampere, Finland.
authors.forename: Pirkko-Liisa
authors.initials: PL
authors.lastname: Kellokumpu-Lehtinen
keywords: Breast cancer; Free breathing; Image guidance; Radiotherapy; Setup errors; Setup margins