title: Robust optimization methods for cardiac sparing in tangential breast IMRT.
abstract: OBJECTIVE
  In left-sided tangential breast intensity modulated radiation therapy (IMRT), the heart may enter the radiation field and receive excessive radiation while the patient is breathing. The patient's breathing pattern is often irregular and unpredictable. We verify the clinical applicability of a heart-sparing robust optimization approach for breast IMRT. We compare robust optimized plans with clinical plans at free-breathing and clinical plans at deep inspiration breath-hold (DIBH) using active breathing control (ABC).
  
  
  METHODS
  Eight patients were included in the study with each patient simulated using 4D-CT. The 4D-CT image acquisition generated ten breathing phase datasets. An average scan was constructed using all the phase datasets. Two of the eight patients were also imaged at breath-hold using ABC. The 4D-CT datasets were used to calculate the accumulated dose for robust optimized and clinical plans based on deformable registration. We generated a set of simulated breathing probability mass functions, which represent the fraction of time patients spend in different breathing phases. The robust optimization method was applied to each patient using a set of dose-influence matrices extracted from the 4D-CT data and a model of the breathing motion uncertainty. The goal of the optimization models was to minimize the dose to the heart while ensuring dose constraints on the target were achieved under breathing motion uncertainty.
  
  
  RESULTS
  Robust optimized plans were improved or equivalent to the clinical plans in terms of heart sparing for all patients studied. The robust method reduced the accumulated heart dose (D10cc) by up to 801 cGy compared to the clinical method while also improving the coverage of the accumulated whole breast target volume. On average, the robust method reduced the heart dose (D10cc) by 364 cGy and improved the optBreast dose (D99%) by 477 cGy. In addition, the robust method had smaller deviations from the planned dose to the accumulated dose. The deviation of the accumulated dose from the planned dose for the optBreast (D99%) was 12 cGy for robust versus 445 cGy for clinical. The deviation for the heart (D10cc) was 41 cGy for robust and 320 cGy for clinical.
  
  
  CONCLUSIONS
  The robust optimization approach can reduce heart dose compared to the clinical method at free-breathing and can potentially reduce the need for breath-hold techniques.
authors.affiliation: Mechanical and Industrial Engineering Department, University of Toronto, Toronto, Ontario M5S 3G8, Canada.
authors.forename: Houra
authors.initials: H
authors.lastname: Mahmoudzadeh
authors.affiliation: Radiation Medicine Program, UHN Princess Margaret Cancer Centre, Toronto, Ontario M5G 2M9, Canada.
authors.forename: Jenny
authors.initials: J
authors.lastname: Lee
authors.affiliation: Mechanical and Industrial Engineering Department, University of Toronto, Toronto, Ontario M5S 3G8, Canada and Techna Institute for the Advancement of Technology for Health, Toronto, Ontario M5G 1P5, Canada.
authors.forename: Timothy C Y
authors.initials: TC
authors.lastname: Chan
authors.affiliation: Radiation Medicine Program, UHN Princess Margaret Cancer Centre, Toronto, Ontario M5G 2M9, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario M5S 3S2, Canada; and Techna Institute for the Advancement of Technology for Health, Toronto, Ontario M5G 1P5, Canada.
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Purdie
mesh_terms: D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000532:radiotherapy*; D062485:Breath Holding; D003198:Computer Simulation; D066264:Datasets as Topic; D056973:Four-Dimensional Computed Tomography; D006321:Heart / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000528:radiation effects*; D006801:Humans; D009038:Motion; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*