title: Continuous monitoring and intrafraction target position correction during treatment improves target coverage for patients undergoing SBRT prostate therapy.
abstract: OBJECTIVE
  To compare the potential benefits of continuous monitoring of prostate position and intervention (CMI) using 2-mm displacement thresholds during stereotactic body radiation therapy (SBRT) treatment to those of a conventional image-guided procedure involving single localization prior to treatment.
  
  
  METHODS
  Eighty-nine patients accrued to a prostate SBRT dose escalation protocol were implanted with radiofrequency transponder beacons. The planning target volume (PTV) margin was 5 mm in all directions, except for 3 mm in the posterior direction. The prostate was kept within 2 mm of its planned position by the therapists halting dose delivery and, if necessary, correcting the couch position. We computed the number, type, and time required for interventions and where the prostate would have been during dose delivery had there been, instead, a single image-guided setup procedure prior to each treatment. Distributions of prostate displacements were computed as a function of time.
  
  
  RESULTS
  After the initial setup, 1.7 interventions per fraction were required, with a concomitant increase in time for dose delivery of approximately 65 seconds. Small systematic drifts in prostate position in the posterior and inferior directions were observed in the study patients. Without CMI, intrafractional motion would have resulted in approximately 10% of patients having a delivered dose that did not meet our clinical coverage requirement, that is, a PTV D95 of >90%. The posterior PTV margin required for 95% of the dose to be delivered with the target positioned within the PTV was computed as a function of time. The margin necessary was found to increase by 2 mm every 5 minutes, starting from the time of the imaging procedure.
  
  
  CONCLUSIONS
  CMI using a tight 2-mm displacement threshold was not only feasible but was found to deliver superior PTV coverage compared with the conventional image-guided procedure in the SBRT setting.
authors: Memorial Sloan Kettering Cancer Center, New York, New York. Electronic address: lovelocm@mskcc.org.
authors: D Michael
authors: DM
authors: Lovelock
authors: Memorial Sloan Kettering Cancer Center, New York, New York.
authors: Alessandra P
authors: AP
authors: Messineo
authors: North Shore-Long Island Jewish Health System, New Hyde Park, New York.
authors: Brett W
authors: BW
authors: Cox
authors: Memorial Sloan Kettering Cancer Center, New York, New York.
authors: Marisa A
authors: MA
authors: Kollmeier
authors: Memorial Sloan Kettering Cancer Center, New York, New York.
authors: Michael J
authors: MJ
authors: Zelefsky
mesh_terms: D019583:Dose Fractionation, Radiation; D005240:Feasibility Studies; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D056888:Patient Positioning / Q000295:instrumentation*; D011471:Prostatic Neoplasms / Q000601:surgery*; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D050397:Radiotherapy, Intensity-Modulated; D013997:Time Factors