title: Dose escalated liver stereotactic body radiation therapy at the mean respiratory position.
abstract: OBJECTIVE
  The dosimetric impact of dose probability based planning target volume (PTV) margins for liver cancer patients receiving stereotactic body radiation therapy (SBRT) was compared with standard PTV based on the internal target volume (ITV). Plan robustness was evaluated by accumulating the treatment dose to ensure delivery of the intended plan.
  
  
  METHODS
  Twenty patients planned on exhale CT for 27 to 50 Gy in 6 fractions using an ITV-based PTV and treated free-breathing were retrospectively evaluated. Isotoxic, dose escalated plans were created on midposition computed tomography (CT), representing the mean breathing position, using a dose probability PTV. The delivered doses were accumulated using biomechanical deformable registration of the daily cone beam CT based on liver targeting at the exhale or mean breathing position, for the exhale and midposition CT plans, respectively.
  
  
  RESULTS
  The dose probability PTVs were on average 38% smaller than the ITV-based PTV, enabling an average ± standard deviation increase in the planned dose to 95% of the PTV of 4.0 ± 2.8 Gy (9 ± 5%) on the midposition CT (P<.01). For both plans, the delivered minimum gross tumor volume (GTV) doses were greater than the planned nominal prescribed dose in all 20 patients and greater than the planned dose to 95% of the PTV in 18 (90%) patients. Nine patients (45%) had 1 or more GTVs with a delivered minimum dose more than 5 Gy higher with the midposition CT plan using dose probability PTV, compared with the delivered dose with the exhale CT plan using ITV-based PTV.
  
  
  CONCLUSIONS
  For isotoxic liver SBRT planned and delivered at the mean respiratory, reduced dose probability PTV enables a mean escalation of 4 Gy (9%) in 6 fractions over ITV-based PTV. This may potentially improve local control without increasing the risk of tumor underdosing.
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada; Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada. Electronic address: michael.velec@rmp.uhn.ca.
authors.forename: Michael
authors.initials: M
authors.lastname: Velec
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada.
authors.forename: Joanne L
authors.initials: JL
authors.lastname: Moseley
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada; Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Laura A
authors.initials: LA
authors.lastname: Dawson
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada; Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Kristy K
authors.initials: KK
authors.lastname: Brock
mesh_terms: D054893:Cone-Beam Computed Tomography; D045853:Exhalation; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology / Q000556:secondary / Q000601:surgery*; D009068:Movement; D011336:Probability; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D012119:Respiration; D012189:Retrospective Studies; D047368:Tumor Burden