title: Magnetic Resonance Image Guided Radiation Therapy for External Beam Accelerated Partial-Breast Irradiation: Evaluation of Delivered Dose and Intrafractional Cavity Motion.
abstract: To use magnetic resonance image guided radiation therapy (MR-IGRT) for accelerated partial-breast irradiation (APBI) to (1) determine intrafractional motion of the breast surgical cavity; and (2) assess delivered dose versus planned dose.
  
  
  
  Thirty women with breast cancer (stages 0-I) who underwent breast-conserving surgery were enrolled in a prospective registry evaluating APBI using a 0.35-T MR-IGRT system. Clinical target volume was defined as the surgical cavity plus a 1-cm margin (excluding chest wall, pectoral muscles, and 5 mm from skin). No additional margin was added for the planning target volume (PTV). A volumetric MR image was acquired before each fraction, and patients were set up to the surgical cavity as visualized on MR imaging. To determine the delivered dose for each fraction, the electron density map and contours from the computed tomography simulation were transferred to the pretreatment MR image via rigid registration. Intrafractional motion of the surgical cavity was determined by applying a tracking algorithm to the cavity contour as visualized on cine MR.
  
  
  
  Median PTV volume was reduced by 52% when using no PTV margin compared with a 1-cm PTV margin used conventionally. The mean (± standard deviation) difference between planned and delivered dose to the PTV (V95) was 0.6% ± 0.1%. The mean cavity displacement in the anterior-posterior and superior-inferior directions was 0.6 ± 0.4 mm and 0.6 ± 0.3 mm, respectively. The mean margin required for at least 90% of the cavity to be contained by the margin for 90% of the time was 0.7 mm (5th-95th percentile: 0-2.7 mm).
  
  
  
  Minimal intrafractional motion was observed, and the mean difference between planned and delivered dose was less than 1%. Assessment of efficacy and cosmesis of this MR-guided APBI approach is under way.
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Sahaja
authors: S
authors: Acharya
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Benjamin W
authors: BW
authors: Fischer-Valuck
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Thomas R
authors: TR
authors: Mazur
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Austen
authors: A
authors: Curcuru
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Karl
authors: K
authors: Sona
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Rojano
authors: R
authors: Kashani
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Olga
authors: O
authors: Green
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Laura
authors: L
authors: Ochoa
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Sasa
authors: S
authors: Mutic
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: Imran
authors: I
authors: Zoberi
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri.
authors: H Harold
authors: HH
authors: Li
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri. Electronic address: mthomas@radonc.wustl.edu.
authors: Maria A
authors: MA
authors: Thomas
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy* / Q000601:surgery; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D019028:Magnetic Resonance Imaging, Cine; D000072662:Margins of Excision; D015412:Mastectomy, Segmental; D008875:Middle Aged; D009038:Motion; D009068:Movement; D058958:Organs at Risk / Q000000981:diagnostic imaging; D011446:Prospective Studies; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D014057:Tomography, X-Ray Computed