title: Impact of geometric uncertainties on evaluation of treatment techniques for prostate cancer.
abstract: OBJECTIVE
  To assess the impact of patient repositioning and internal organ motion on prostate treatment plans using three-dimensional conformal and intensity-modulated radiotherapy.
  
  
  METHODS
  Four-field, six-field, and simplified intensity-modulated arc therapy plans were generated for 5 prostate cancer patients. The planning target volume was created by adding a 1-cm margin to the clinical target volume. A convolution model was used to estimate the effect of random geometric uncertainties during treatment. Dose statistics, tumor control probabilities, and normal tissue complication probabilities were compared with and without the presence of uncertainty. The impact of systematic uncertainties was also investigated.
  
  
  RESULTS
  Compared with the planned treatments, the delivered dose distribution with random geometric uncertainties displayed an increase in the apparent minimal dose to the prostate and seminal vesicles and a decrease in the rectal volume receiving a high dose. This increased the tumor control probabilities and decreased the normal tissue complication probabilities. Changes were seen in the percentage of prostate volume receiving 100% and 95% of the prescribed dose, and the minimal dose and tumor control probabilities for the target volume. In addition, the volume receiving at least 65 Gy, the minimal dose, and normal tissue complication probabilities changed considerably for the rectum. The simplified intensity-modulated arc therapy technique was the most sensitive to systematic errors, especially in the anterior-posterior and superior-inferior directions.
  
  
  CONCLUSIONS
  Geometric uncertainties should be considered when evaluating treatment plans. Contrary to the widely held belief, increased conformation of the dose distribution is not always associated with increased sensitivity to random geometric uncertainties if a sufficient planning target volume margin is used. Systematic errors may have a variable effect, depending on the treatment technique used.
authors.affiliation: Radiation Treatment Program, London Regional Cancer Program, London Health Sciences Centre, London, ON, Canada.
authors.forename: Tim
authors.initials: T
authors.lastname: Craig
authors.forename: Eugene
authors.initials: E
authors.lastname: Wong
authors.forename: Glenn
authors.initials: G
authors.lastname: Bauman
authors.forename: Jerry
authors.initials: J
authors.lastname: Battista
authors.forename: Jake
authors.initials: J
authors.lastname: Van Dyk
mesh_terms: D000465:Algorithms; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum; D012669:Seminal Vesicles; D035501:Uncertainty