title: Set-up uncertainties: online correction with X-ray volume imaging.
abstract: OBJECTIVE
  To determine interfractional three-dimensional set-up errors using X-ray volumetric imaging (XVI).
  
  
  METHODS
  Between December 2007 and August 2009, 125 patients were taken up for image-guided radiotherapy using online XVI. After matching of reference and acquired volume view images, set-up errors in three translation directions were recorded and corrected online before treatment each day. Mean displacements, population systematic (Σ), and random (σ) errors were calculated and analyzed using SPSS (v16) software. Optimum clinical target volume (CTV) to planning target volume (PTV) margin was calculated using Van Herk's (2.5Σ + 0.7 σ) and Stroom's (2Σ + 0.7 σ) formula.
  
  
  RESULTS
  Patients were grouped in 4 cohorts, namely brain, head and neck, thorax, and abdomen-pelvis. The mean vector displacement recorded were 0.18 cm, 0.15 cm, 0.36 cm, and 0.35 cm for brain, head and neck, thorax, and abdomen-pelvis, respectively. Analysis of individual mean set-up errors revealed good agreement with the proposed 0.3 cm isotropic margins for brain and 0.5 cm isotropic margins for head-neck. Similarly, 0.5 cm circumferential and 1 cm craniocaudal proposed margins were in agreement with thorax and abdomen-pelvic cases.
  
  
  CONCLUSIONS
  The calculated mean displacements were well within CTV-PTV margin estimates of Van Herk (90% population coverage to minimum 95% prescribed dose) and Stroom (99% target volume coverage by 95% prescribed dose). Employing these individualized margins in a particular cohort ensure comparable target coverage as described in literature, which is further improved if XVI-aided set-up error detection and correction is used before treatment.
authors.affiliation: Division of Radiation Oncology, Medanta Cancer Institute, Medanta-The Medicity, Sector-38, Gurgaon, Haryana 122 001, India.
authors.forename: Tejinder
authors.initials: T
authors.lastname: Kataria
authors.forename: Ashu
authors.initials: A
authors.lastname: Abhishek
authors.forename: Pranav
authors.initials: P
authors.lastname: Chadha
authors.forename: Janardhan
authors.initials: J
authors.lastname: Nandigam
mesh_terms: D000008:Abdominal Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D001932:Brain Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D005240:Feasibility Studies; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D006801:Humans; D009862:Online Systems; D010386:Pelvic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D011379:Prognosis; D011446:Prospective Studies; D013902:Radiography, Thoracic; D011880:Radiotherapy Planning, Computer-Assisted; D014965:X-Rays