title: Comparison of setup accuracy of three different image assessment methods for tangential breast radiotherapy.
abstract: BACKGROUND
  To compare the differences in setup errors measured with electronic portal image (EPI) and cone-beam computed tomography (CBCT) in patients undergoing tangential breast radiotherapy (RT). Relationship between setup errors, body mass index (BMI) and breast size was assessed.
  
  
  METHODS
  Twenty-five patients undergoing postoperative RT to the breast were consented for this study. Weekly CBCT scans were acquired and retrospectively registered to the planning CT in three dimensions, first using bony anatomy for bony registration (CBCT-B) and again using breast tissue outline for soft tissue registration (CBCT-S). Digitally reconstructed radiographs (DRR) generated from CBCT to simulate EPI were compared to the planning DRR using bony anatomy in the V (parallel to the cranio-caudal axis) and U (perpendicular to V) planes. The systematic (Σ) and random (σ) errors were calculated and correlated with BMI and breast size.
  
  
  RESULTS
  The systematic and random errors for EPI (ΣV= 3.7 mm, ΣU= 2.8 mm and σV= 2.9 mm, σU= 2.5) and CBCT-B (ΣV= 3.5 mm, ΣU= 3.4 mm and σV= 2.8 mm, σU= 2.8) were of similar magnitude in the V and U planes. Similarly, the differences in setup errors for CBCT-B and CBCT-S in three dimensions were less than 1 mm. Only CBCT-S setup error correlated with BMI and breast size.
  
  
  CONCLUSIONS
  CBCT and EPI show insignificant variation in their ability to detect setup error. These findings suggest no significant differences that would make one modality considered superior over the other and EPI should remain the standard of care for most patients. However, there is a correlation with breast size, BMI and setup error as detected by CBCT-S, justifying the use of CBCT-S for larger patients.
authors.affiliation: Liverpool Cancer Therapy Centre, Liverpool, New South Wales, Australia.|Ingham Institute for Applied Medical Research, Liverpool, New South Wales, Australia.|South Western Clinical School, University of New South Wales, Liverpool, New South Wales, Australia.
authors.forename: Vikneswary
authors.initials: V
authors.lastname: Batumalai
authors.affiliation: Liverpool Cancer Therapy Centre, Liverpool, New South Wales, Australia.
authors.forename: Penny
authors.initials: P
authors.lastname: Phan
authors.affiliation: Liverpool Cancer Therapy Centre, Liverpool, New South Wales, Australia.
authors.forename: Callie
authors.initials: C
authors.lastname: Choong
authors.affiliation: Liverpool Cancer Therapy Centre, Liverpool, New South Wales, Australia.|Ingham Institute for Applied Medical Research, Liverpool, New South Wales, Australia.|South Western Clinical School, University of New South Wales, Liverpool, New South Wales, Australia.|Centre for Medical Radiation Physics, University of Wollongong, Sydney, New South Wales, Australia.|School of Physics, University of Sydney, Liverpool, New South Wales, Australia.
authors.forename: Lois
authors.initials: L
authors.lastname: Holloway
authors.affiliation: Liverpool Cancer Therapy Centre, Liverpool, New South Wales, Australia.|Ingham Institute for Applied Medical Research, Liverpool, New South Wales, Australia.|South Western Clinical School, University of New South Wales, Liverpool, New South Wales, Australia.|School of Medicine, University of Western Sydney, New South Wales, Australia.
authors.forename: Geoff P
authors.initials: GP
authors.lastname: Delaney
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D054893:Cone-Beam Computed Tomography / Q000379:methods / Q000592:standards*; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods / Q000592:standards*; D007091:Image Processing, Computer-Assisted / Q000379:methods / Q000592:standards*; D008875:Middle Aged; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*
keywords: Breast cancer; cone-beam computed tomography (CBCT); electronic portal image (EPI); radiotherapy; setup errors