title: Does IGRT ensure target dose coverage of head and neck IMRT patients?
abstract: OBJECTIVE
  To determine if image-guided radiotherapy (IGRT) ensures dose coverage to the target, and to assess the dosimetric impact of anatomic changes using megavoltage cone-beam CT (MVCBCT) for patient positioning during head and neck IMRT.
  
  
  METHODS
  Forty-eight MVCBCT from 10 head and neck IMRT/IGRT patients were analyzed off-line. Target volumes and organs at risk (OARs) contours delineated on CT were transferred and adjusted on MVCBCT images. Each MVCBCT was processed to allow dose recalculation, resulting in 469 dose-volume histograms (DVHs). The concept of dosimetric latitude was introduced to provide a clinical perspective.
  
  
  RESULTS
  MVCBCT target DVHs showed a moderate level of difference in D95 (dose to ≥95% of volume), generally less than a 5% difference from the planned dose. Delivered-dose increases to the spinal cord and brainstem showed no apparent time trend. The 4mm margin around OARs was a useful precaution to prevent exceeding critical dose thresholds. The parotid glands showed progressive increases in mean dose related to shrinkage of the external contours.
  
  
  CONCLUSIONS
  IGRT repositioning ensured target volume coverage, but significant dose variations were observed for OARs. The dosimetric impact of anatomic changes during radiotherapy was of lesser importance than the effects of IGRT repositioning.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, CA 94143, USA.
authors.forename: Pierre
authors.initials: P
authors.lastname: Graff
authors.forename: Weigang
authors.initials: W
authors.lastname: Hu
authors.forename: Sue S
authors.initials: SS
authors.lastname: Yom
authors.forename: Jean
authors.initials: J
authors.lastname: Pouliot
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D056888:Patient Positioning; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D047368:Tumor Burden