title: Dosimetric impact of setup accuracy for an electron breast boost technique.
abstract: OBJECTIVE
  To determine the setup error on an electron breast boost technique using daily cone beam computed tomography (CBCT). Patient and setup attributes were studied as contributing factors to the accuracy.
  
  
  METHODS
  Reproducibility of a modified lateral decubitus position breast boost setup was verified for 33 patients using CBCT. Three-dimensional matching was performed between the CBCT and the initial planning CT for each boost fraction by matching the tumor bed and/or surgical clips. The dosimetric impact of the daily positioning error was achieved by rerunning the initial treatment plans incorporating the recorded shifts to study the dose differences. Breast compression, decubitus angle, tumor bed location and volume, and cup size were studied for their contribution to setup error.
  
  
  RESULTS
  The range of setup errors was: 1.5 cm anterior to 9 mm posterior, 1.3 cm superior to 2.3 cm inferior, and 3.2 cm medial to 2.4 cm lateral. Seven patients had setup errors that were ≥2-cm margin placed on the tumor bed and scar. Four of those 7 patients had unacceptable coverage as defined by the volume of the tumor bed plus scar that is covered by the 90% isodose line (V90) compared with the original plan. All other patients had no discernible difference in the coverage (V90). The use of compression, tumor bed location, or volumes >20 mL showed no effect on coverage.
  
  
  CONCLUSIONS
  In general, this study supported that a 2-cm margin was adequate (29 of 33 patients) when patients are treated under typical conditions. Care should be taken when high electron energies are selected because the coverage at depth is more difficult to maintain in the clinical environment.
authors: Department of Radiation Oncology, The University of Texas Medical Branch, Galveston, Texas. Electronic address: scott.davidson@utmb.edu.
authors: Scott
authors: S
authors: Davidson
authors: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
authors: Steven
authors: S
authors: Kirsner
authors: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
authors: Bryan
authors: B
authors: Mason
authors: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
authors: Kelly
authors: K
authors: Kisling
authors: Department of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
authors: Renee D
authors: RD
authors: Barrett
authors: Department of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
authors: Anthony
authors: A
authors: Bonetati
authors: Department of Radiation Oncology, The University of Tennessee Health Science Center, Memphis, Tennessee.
authors: Matthew T
authors: MT
authors: Ballo
mesh_terms: D000328:Adult; D000368:Aged; D001940:Breast / Q000473:pathology*; D001943:Breast Neoplasms / Q000532:radiotherapy*; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005260:Female; D006801:Humans; D008875:Middle Aged; D056888:Patient Positioning / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*