title: Prospective study of cone-beam computed tomography image-guided radiotherapy for prone accelerated partial breast irradiation.
abstract: OBJECTIVE
  To report setup variations during prone accelerated partial breast irradiation (APBI).
  
  
  METHODS
  New York University (NYU) 07-582 is an institutional review board-approved protocol of cone-beam computed tomography (CBCT) to deliver image-guided ABPI in the prone position. Eligible are postmenopausal women with pT1 breast cancer excised with negative margins and no nodal involvement. A total dose of 30 Gy in five daily fractions of 6 Gy are delivered to the planning target volume (the tumor cavity with 1.5-cm margin) by image-guided radiotherapy. Patients are set up prone, on a dedicated mattress, used for both simulation and treatment. After positioning with skin marks and lasers, CBCTs are performed and the images are registered to the planning CT. The resulting shifts (setup corrections) are recorded in the three principal directions and applied. Portal images are taken for verification. If they differ from the planning digital reconstructed radiographs, the patient is reset, and a new CBCT is taken.
  
  
  RESULTS
  70 consecutive patients have undergone a total of 343 CBCTs: 7 patients had four of five planned CBCTs performed. Seven CBCTs (2%) required to be repeated because of misalignment in the comparison between portal and digital reconstructed radiograph image after the first CBCT. The mean shifts and standard deviations in the anterior-posterior (AP), superior-inferior (SI), and medial-lateral (ML) directions were -0.19 (0.54), -0.02 (0.33), and -0.02 (0.43) cm, respectively. The average root mean squares of the daily shifts were 0.50 (0.28), 0.29 (0.17), and 0.38 (0.20). A conservative margin formula resulted in a recommended margin of 1.26, 0.73, 0.96 cm in the AP, SI, and ML directions.
  
  
  CONCLUSIONS
  CBCTs confirmed that the NYU prone APBI setup and treatment technique are reproducible, with interfraction variation comparable to those reported for supine setup. The currently applied margin (1.5 cm) adequately compensates for the setup variation detected.
authors.affiliation: Department of Radiation Oncology, New York University Medical Center, New York, NY 10016, USA. gabor.jozsef@nyumc.org
authors.forename: Gabor
authors.initials: G
authors.lastname: Jozsef
authors.forename: J Keith
authors.initials: JK
authors.lastname: DeWyngaert
authors.forename: Stewart J
authors.initials: SJ
authors.lastname: Becker
authors.forename: Stella
authors.initials: S
authors.lastname: Lymberis
authors.forename: Silvia C
authors.initials: SC
authors.lastname: Formenti
mesh_terms: D000465:Algorithms; D001513:Beds; D001940:Breast / Q000473:pathology; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D005260:Female; D057918:Fiducial Markers; D006801:Humans; D009519:New York City; D009929:Organ Size; D056888:Patient Positioning / Q000295:instrumentation / Q000379:methods*; D016684:Prone Position; D011446:Prospective Studies; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D014495:Universities