title: Reliability of portal control procedure in irradiation of breast cancer: a Bayesian analysis.
abstract: OBJECTIVE
  To determine whether the information gathered from a fixed number of periodic verification films accurately reflects the true imprecision in patient positioning during the whole radiation therapy of early breast cancer.
  
  
  METHODS
  A total of 204 medial and lateral treatment fields were evaluated in 102 breast cancer patients treated with conservative surgery and radiation therapy. For each treatment field, the central lung distance was measured on portal films obtained from one simulation and four treatment controls at weekly intervals during breast irradiation. Systematic and random errors in patient positioning throughout all treatment fractions were estimated from the available controls using Bayesian methods.
  
  
  RESULTS
  The average systematic and random errors during treatment controls were 2.7 and 1.9 mm, respectively. For these mean control values, the probabilities that the true systematic and random errors remain below 5mm during all treatment fractions were 99 and 100%, respectively.
  
  
  CONCLUSIONS
  Reproducibility of patient positioning was supported by a virtually null probability for systematic or random errors greater than 5 mm during the whole radiation therapy. Weekly verification films seem to be sufficient to estimate patient positioning errors with high accuracy in radiotherapy of early breast cancer.
authors.affiliation: Department of Radiation Oncology, Txagorritxu Hospital, E-01009 Vitoria, Spain. aalia@htxa.osakidetza.net
authors.forename: Avelino
authors.initials: A
authors.lastname: Alía
authors.forename: Javier
authors.initials: J
authors.lastname: Mar
authors.forename: Roberto
authors.initials: R
authors.lastname: Pastor-Barriuso
mesh_terms: D000328:Adult; D000368:Aged; D001499:Bayes Theorem; D001943:Breast Neoplasms / Q000532:radiotherapy* / Q000601:surgery; D005260:Female; D006801:Humans; D008875:Middle Aged; D011187:Posture; D011874:Radiometry; D015203:Reproducibility of Results