title: The impact of geometric uncertainty on hypofractionated external beam radiation therapy of prostate cancer.
abstract: OBJECTIVE
  Recent publications indicate alpha/beta for prostate carcinoma could be lower than assumed. Therefore, hypofractionation might increase the therapeutic ratio. However, patient repositioning and organ motion may affect hypofractionated treatments more than conventional treatments. Our purpose is to evaluate the potential impact of geometric uncertainties on hypofractionated treatments.
  
  
  METHODS
  Tumor control probability (TCP) and normal tissue complication probability (NTCP) are calculated for simulated conventional and hypofractionated treatments, assuming alpha/beta of 1.5 Gy for prostate and 3.0 Gy for rectum. A Monte Carlo simulation randomly samples systematic and random displacements and produces the cumulative dose distribution for the prostate and rectum. The limiting number of fractions and the impact of different alpha/beta values are also explored.
  
  
  RESULTS
  A consistent but small reduction in TCP is seen with hypofractionation (generally <1%) as a result of geometric uncertainties. Escalated hypofractionation seems to allow large TCP gains ( approximately 20%) without increasing NTCP. Treatments of five fractions seem to affect outcome minimally. The alpha/beta value has a much greater impact on TCP than geometric uncertainties.
  
  
  CONCLUSIONS
  The potential increased influence of geometric uncertainties on hypofractionation seems small. Limited knowledge of radiobiologic response is likely a greater obstacle to prostate hypofractionation than geometric uncertainties.
authors.affiliation: Radiation Oncology Program, London Regional Cancer Centre, London, Ontario, Canada.
authors.forename: Tim
authors.initials: T
authors.lastname: Craig
authors.forename: Vitali
authors.initials: V
authors.lastname: Moiseenko
authors.forename: Jerry
authors.initials: J
authors.lastname: Battista
authors.forename: Jake
authors.initials: J
authors.lastname: Van Dyk
mesh_terms: D000465:Algorithms; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D011336:Probability; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D012007:Rectum; D012669:Seminal Vesicles