title: Impact of intrafraction and residual interfraction effect on prostate proton pencil beam scanning.
abstract: OBJECTIVE
  To quantitatively evaluate the impact of interplay effect and plan robustness associated with intrafraction and residual interfraction prostate motion for pencil beam scanning proton therapy.
  
  
  METHODS
  Ten prostate cancer patients with weekly verification CTs underwent pencil beam scanning with the bilateral single-field uniform dose (SFUD) modality. A typical field had 10-15 energy layers and 500-1000 spots. According to their treatment logs, each layer delivery time was <1 s, with average time to change layers of approximately 8 s. Real-time intrafraction prostate motion was determined from our previously reported prospective study using Calypso beacon transponders. Prostate motion and beam delivering sequence of the worst-case scenario patient were synchronized to calculate the "true" dose received by the prostate. The intrafraction effect was examined by applying the worst-case scenario prostate motion on the planning CT, and the residual interfraction effect was examined on the basis of weekly CT scans. The resultant dose variation of target and critical structures was examined to evaluate the interplay effect.
  
  
  RESULTS
  The clinical target volume (CTV) coverage was degraded because of both effects. The CTV D99 (percentage dose to 99% of the CTV) varied up to 10% relative to the initial plan in individual fractions. However, over the entire course of treatment the total dose degradation of D99 was 2%-3%, with a standard deviation of <2%. Absolute differences between SFUD, intensity modulate proton therapy, and one-field-per-day SFUD plans were small. The intrafraction effect dominated over the residual interfraction effect for CTV coverage. Mean dose to the anterior rectal wall increased approximately 10% because of combined residual interfraction and intrafraction effects, the interfraction effect being dominant.
  
  
  CONCLUSIONS
  Both intrafraction and residual interfraction prostate motion degrade CTV coverage within a clinically acceptable level. One-field-per-day SFUD delivered twice is as robust as the bilateral SFUD plan treated daily over the course of treatment.
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania; ProCure Proton Therapy Center, Somerset, New Jersey. Electronic address: shktang@gmail.com.
authors.forename: Shikui
authors.initials: S
authors.lastname: Tang
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Curtiland
authors.initials: C
authors.lastname: Deville
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Zelig
authors.initials: Z
authors.lastname: Tochner
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania; Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Baltimore, Maryland.
authors.forename: Ken Kang-Hsin
authors.initials: KK
authors.lastname: Wang
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: James
authors.initials: J
authors.lastname: McDonough
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Neha
authors.initials: N
authors.lastname: Vapiwala
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Stefan
authors.initials: S
authors.lastname: Both
mesh_terms: D059925:Anatomic Landmarks; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009068:Movement; D058958:Organs at Risk / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D061766:Proton Therapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects; D012062:Relative Biological Effectiveness; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects