title: Portal imaging based definition of the planning target volume during pelvic irradiation for gynecological malignancies.
abstract: OBJECTIVE
  Geometrical accuracy in patient positioning can vary substantially during external radiotherapy. This study estimated the set-up accuracy during pelvic irradiation for gynecological malignancies for determination of safety margins (planning target volume, PTV).
  
  
  METHODS
  Based on electronic portal imaging devices (EPID), 25 patients undergoing 4-field pelvic irradiation for gynecological malignancies were analyzed with regard to set-up accuracy during the treatment course. Regularly performed EPID images were used in order to systematically assess the systematic and random component of set-up displacements. Anatomical matching of verification and simulation images was followed by measuring corresponding distances between the central axis and anatomical features. Data analysis of set-up errors referred to the x-, y-,and z-axes. Additionally, cumulative frequencies were evaluated.
  
  
  RESULTS
  A total of 50 simulation films and 313 verification images were analyzed. For the anterior-posterior (AP) beam direction mean deviations along the x- and z-axes were 1.5 mm and -1.9 mm, respectively. Moreover, random errors of 4.8 mm (x-axis) and 3.0 mm (z-axis) were determined. Concerning the latero-lateral treatment fields, the systematic errors along the two axes were calculated to 2.9 mm (y-axis) and -2.0 mm (z-axis) and random errors of 3.8 mm and 3.5 mm were found, respectively. The cumulative frequency of misalignments < or =5 mm showed values of 75% (AP fields) and 72% (latero-lateral fields). With regard to cumulative frequencies < or =10 mm quantification revealed values of 97% for both beam directions.
  
  
  CONCLUSIONS
  During external pelvic irradiation therapy for gynecological malignancies, EPID images on a regular basis revealed acceptable set-up inaccuracies. Safety margins (PTV) of 1 cm appear to be sufficient, accounting for more than 95% of all deviations.
authors.affiliation: Department of Radiotherapy and Radiobiology, University Hospital Vienna, Austria.
authors.forename: U
authors.initials: U
authors.lastname: Mock
authors.forename: K
authors.initials: K
authors.lastname: Dieckmann
authors.forename: U
authors.initials: U
authors.lastname: Wolff
authors.forename: T H
authors.initials: TH
authors.lastname: Knocke
authors.forename: R
authors.initials: R
authors.lastname: Pötter
mesh_terms: D001918:Brachytherapy; D019583:Dose Fractionation, Radiation; D005260:Female; D005833:Genital Neoplasms, Female / Q000532:radiotherapy*; D006801:Humans; D055585:Physical Phenomena; D010825:Physics; D020266:Radiotherapy, Conformal / Q000379:methods* / Q000592:standards; D012107:Research Design; D012189:Retrospective Studies