title: Impact of the frequency of online verifications on the patient set-up accuracy and set-up margins.
abstract: OBJECTIVE
  The purpose of the study was to evaluate the patient set-up error of different anatomical sites, to estimate the effect of different frequencies of online verifications on the patient set-up accuracy, and to calculate margins to accommodate for the patient set-up error (ICRU set-up margin, SM).
  
  
  METHODS
  Alignment data of 148 patients treated with inversed planned intensity modulated radiotherapy (IMRT) or three-dimensional conformal radiotherapy (3D-CRT) of the head and neck (n = 31), chest (n = 72), abdomen (n = 15), and pelvis (n = 30) were evaluated. The patient set-up accuracy was assessed using orthogonal megavoltage electronic portal images of 2328 fractions of 173 planning target volumes (PTV). In 25 patients, two PTVs were analyzed where the PTVs were located in different anatomical sites and treated in two different radiotherapy courses. The patient set-up error and the corresponding SM were retrospectively determined assuming no online verification, online verification once a week and online verification every other day.
  
  
  RESULTS
  The SM could be effectively reduced with increasing frequency of online verifications. However, a significant frequency of relevant set-up errors remained even after online verification every other day. For example, residual set-up errors larger than 5 mm were observed on average in 18% to 27% of all fractions of patients treated in the chest, abdomen and pelvis, and in 10% of fractions of patients treated in the head and neck after online verification every other day.
  
  
  CONCLUSIONS
  In patients where high set-up accuracy is desired, daily online verification is highly recommended.
authors.affiliation: Department of Radiation Oncology, Saad Specialist Hospital, Al Khobar, Saudi Arabia. volker.rudat@saad.com.sa
authors.forename: Volker
authors.initials: V
authors.lastname: Rudat
authors.forename: Mohamed
authors.initials: M
authors.lastname: Hammoud
authors.forename: Yogin
authors.initials: Y
authors.lastname: Pillay
authors.forename: Abdul Aziz
authors.initials: AA
authors.lastname: Alaradi
authors.forename: Adel
authors.initials: A
authors.lastname: Mohamed
authors.forename: Saleh
authors.initials: S
authors.lastname: Altuwaijri
mesh_terms: D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000379:methods; D020407:Internet; D015233:Models, Statistical; D010315:Particle Accelerators; D049268:Positron-Emission Tomography / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D015203:Reproducibility of Results; D012984:Software; D014057:Tomography, X-Ray Computed / Q000379:methods