title: Effect of intrafraction prostate motion on proton pencil beam scanning delivery: a quantitative assessment.
abstract: OBJECTIVE
  To assess the dosimetric impact caused by the interplay between intrafraction prostate motion and the intermittent delivery of proton pencil beam scanning (PBS).
  
  
  METHODS
  A cohort of 10 prostate patients was treated with PBS using a bilateral single-field uniform dose (SFUD) modality. Bilateral intensity-modulated proton therapy (IMPT) plans were generated for comparison. Because beam-on time in PBS was intermittent, the actual beam-on time was determined from treatment logs. Prostate motion was generalized according to real-time Calypso tracking data from our previously reported prospective photon trial. We investigated potential dose deviations by considering the interplay effect resulting from the worst-case scenario motion and the PBS delivery sequence.
  
  
  RESULTS
  For both bilateral-field SFUD and IMPT plans, clinical target volume (CTV) D99% coverage was degraded <2% owing to prostate intrafraction motion when averaged over the course of treatment, but was >10% for the worst fraction. The standard deviation of CTV D99% distribution was approximately 1.2%. The CTV coverage of individual fields in SFUD plans degraded as time elapsed after the initial alignment, owing to prostate drift. Intensity-modulated proton therapy and SFUD demonstrated comparable results when bilateral opposed fields were used. Single-field SFUD plans that were repainted twice, which could reduce half of the treatment time, resulted in similar CTV coverage as bilateral-field plans.
  
  
  CONCLUSIONS
  Intrafraction prostate motion affects the actual delivered dose to CTV; however, when averaged over the course of treatment, CTV D99% coverage degraded only approximately 2% even for the worst-case scenario. The IMPT plan results are comparable to those of the SFUD plan, and similar coverage can be achieved if treated by SFUD 1 lateral field per day when rescanning the field twice to shorten the treatment time and mitigate intrafraction motion.
authors: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. TangS@uphs.upenn.edu
authors: Shikui
authors: S
authors: Tang
authors: Curtiland
authors: C
authors: Deville
authors: James
authors: J
authors: McDonough
authors: Zelig
authors: Z
authors: Tochner
authors: Ken Kang-Hsin
authors: KK
authors: Wang
authors: Neha
authors: N
authors: Vapiwala
authors: Stefan
authors: S
authors: Both
mesh_terms: D000465:Algorithms; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D061766:Proton Therapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D058998:Remote Sensing Technology / Q000379:methods*; D013997:Time Factors