title: Dosimetric and radiobiological consequences of computed tomography-guided adaptive strategies for intensity modulated radiation therapy of the prostate.
abstract: OBJECTIVE
  To examine a range of scenarios for image-guided adaptive radiation therapy of prostate cancer, including different schedules for megavoltage CT imaging, patient repositioning, and dose replanning.
  
  
  METHODS
  We simulated multifraction dose distributions with deformable registration using 35 sets of megavoltage CT scans of 13 patients. We computed cumulative dose-volume histograms, from which tumor control probabilities and normal tissue complication probabilities (NTCPs) for rectum were calculated. Five-field intensity modulated radiation therapy (IMRT) with 18-MV x-rays was planned to achieve an isocentric dose of 76 Gy to the clinical target volume (CTV). The differences between D95, tumor control probability, V70Gy, and NTCP for rectum, for accumulated versus planned dose distributions, were compared for different target volume sizes, margins, and adaptive strategies.
  
  
  RESULTS
  The CTV D95 for IMRT treatment plans, averaged over 13 patients, was 75.2 Gy. Using the largest CTV margins (10/7 mm), the D95 values accumulated over 35 fractions were within 2% of the planned value, regardless of the adaptive strategy used. For tighter margins (5 mm), the average D95 values dropped to approximately 73.0 Gy even with frequent repositioning, and daily replanning was necessary to correct this deficit. When personalized margins were applied to an adaptive CTV derived from the first 6 treatment fractions using the STAPLE (Simultaneous Truth and Performance Level Estimation) algorithm, target coverage could be maintained using a single replan 1 week into therapy. For all approaches, normal tissue parameters (rectum V(70Gy) and NTCP) remained within acceptable limits.
  
  
  CONCLUSIONS
  The frequency of adaptive interventions depends on the size of the CTV combined with target margins used during IMRT optimization. The application of adaptive target margins (<5 mm) to an adaptive CTV determined 1 week into therapy minimizes the need for subsequent dose replanning.
authors: Department of Medical Biophysics, Western University, London, ON, Canada; Department of Oncology, Western University, London, ON, Canada; London Regional Cancer Program, London Health Sciences Centre, London, ON, Canada. Electronic address: j2b@uwo.ca.
authors: Jerry J
authors: JJ
authors: Battista
authors: Carol
authors: C
authors: Johnson
authors: David
authors: D
authors: Turnbull
authors: Jeff
authors: J
authors: Kempe
authors: Karl
authors: K
authors: Bzdusek
authors: Jacob
authors: J
authors: Van Dyk
authors: Glenn
authors: G
authors: Bauman
mesh_terms: D019583:Dose Fractionation, Radiation; D005269:Femur / Q000000981:diagnostic imaging / Q000528:radiation effects; D006801:Humans; D008297:Male; D059351:Organ Sparing Treatments / Q000379:methods; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects*; D056888:Patient Positioning; D011467:Prostate / Q000000981:diagnostic imaging / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000009:adverse effects / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects*; D013997:Time Factors; D014057:Tomography, X-Ray Computed / Q000379:methods; D047368:Tumor Burden; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects