title: The dosimetric impact of prostate rotations during electromagnetically guided external-beam radiation therapy.
abstract: OBJECTIVE
  To study the impact of daily rotations and translations of the prostate on dosimetric coverage during radiation therapy (RT).
  
  
  METHODS
  Real-time tracking data for 26 patients were obtained during RT. Intensity modulated radiation therapy plans meeting RTOG 0126 dosimetric criteria were created with 0-, 2-, 3-, and 5-mm planning target volume (PTV) margins. Daily translations and rotations were used to reconstruct prostate delivered dose from the planned dose. D95 and V79 were computed from the delivered dose to evaluate target coverage and the adequacy of PTV margins. Prostate equivalent rotation is a new metric introduced in this study to quantify prostate rotations by accounting for prostate shape and length of rotational lever arm.
  
  
  RESULTS
  Large variations in prostate delivered dose were seen among patients. Adequate target coverage was met in 39%, 65%, and 84% of the patients for plans with 2-, 3-, and 5-mm PTV margins, respectively. Although no correlations between prostate delivered dose and daily rotations were seen, the data showed a clear correlation with prostate equivalent rotation.
  
  
  CONCLUSIONS
  Prostate rotations during RT could cause significant underdosing even if daily translations were managed. These rotations should be managed with rotational tolerances based on prostate equivalent rotations.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109, USA. hanan.amro@gmail.com
authors.forename: Hanan
authors.initials: H
authors.lastname: Amro
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Hamstra
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Mcshan
authors.forename: Howard
authors.initials: H
authors.lastname: Sandler
authors.forename: Karen
authors.initials: K
authors.lastname: Vineberg
authors.forename: Scott
authors.initials: S
authors.lastname: Hadley
authors.forename: Dale
authors.initials: D
authors.lastname: Litzenberg
mesh_terms: D004574:Electromagnetic Fields; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000033:anatomy & histology / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012399:Rotation; D047368:Tumor Burden