title: The dosimetric effect of intrafraction prostate motion on step-and-shoot intensity-modulated radiation therapy plans: magnitude, correlation with motion parameters, and comparison with helical tomotherapy plans.
abstract: OBJECTIVE
  To determine the daily and cumulative dosimetric effects of intrafraction prostate motion on step-and-shoot (SNS) intensity-modulated radiation therapy (IMRT) plans, to evaluate the correlation of dosimetric effect with motion-based metrics, and to compare on a fraction-by-fraction basis the dosimetric effect induced in SNS and helical tomotherapy plans.
  
  
  METHODS
  Intrafraction prostate motion data from 486 fractions and 15 patients were available. A motion-encoded dose calculation technique was used to determine the variation of the clinical target volume (CTV) D(95%) values with respect to the static plan for SNS plans. The motion data were analyzed separately, and the correlation coefficients between various motion-based metrics and the dosimetric effect were determined. The dosimetric impact was compared with that incurred during another IMRT technique to assess correlation across different delivery techniques.
  
  
  RESULTS
  The mean (±1 standard deviation [SD]) change in D(95%) in the CTV over all 486 fractions was 0.2 ± 0.5%. After the delivery of five and 12 fractions, the mean (±1 SD) changes over the 15 patients in CTV D(95%) were 0.0 ± 0.2% and 0.1 ± 0.2%, respectively. The correlation coefficients between the CTV D(95%) changes and the evaluated motion metrics were, in general, poor and ranged from r = -0.2 to r = -0.39. Dosimetric effects introduced by identical motion in SNS and helical tomotherapy IMRT techniques were poorly correlated with a correlation coefficient of r = 0.32 for the CTV.
  
  
  CONCLUSIONS
  The dosimetric impact of intrafraction prostate motion on the CTV is, in general, small. In only 4% of all fractions did the dosimetric consequence exceed 1% in the CTV. As expected, the cumulative effect was further reduced with fractionation. The poor correlations between the calculated motion parameters and the subsequent dosimetric effect implies that motion-based thresholds are of limited value in predicting the dosimetric impact of intrafraction motion. The dosimetric effects between the two evaluated delivery techniques were poorly correlated.
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center Orlando, Orlando, Florida, USA. Katja.Langen@orlandohealth.com
authors.forename: Katja M
authors.initials: KM
authors.lastname: Langen
authors.forename: Bhavin
authors.initials: B
authors.lastname: Chauhan
authors.forename: Jeffrey V
authors.initials: JV
authors.lastname: Siebers
authors.forename: Joseph
authors.initials: J
authors.lastname: Moore
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Kupelian
mesh_terms: D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009038:Motion; D009068:Movement; D010315:Particle Accelerators; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*