title: The ratio of weight loss to planning target volume significantly impacts setup errors in nasopharyngeal cancer patients undergoing helical tomotherapy with daily megavoltage computed tomography.
abstract: BACKGROUND
  Changes in head and neck anatomy during radiation therapy (RT) produce setup uncertainties of nasopharyngeal cancer (NPC) irradiation. We retrospectively analyzed image guidance data to identify clinical predictors of setup errors.
  
  
  METHODS
  The data of 217 NPC patients undergoing definitive RT on a helical tomotherapy (HT) unit were analyzed. Factors including tumor stage, body mass index, weight loss, and planning target volume (PTV) were assessed as predictors of daily megavoltage computed tomography (MVCT) setup displacements, which were automatically registered using software.
  
  
  RESULTS
  Mean daily setup displacements (in mm) were 1.2 ± 0.6, 1.8 ± 0.8, 3.4 ± 1.4 in the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions, respectively. Mean weight loss was 4.6 ± 3.3 kg (6.8 ± 4.9%). Patients with weight loss > 5% had significantly larger setup displacements in the AP (3.6 ± 1.5vs. 2.9 ± 1.1 mm, p < 0.001) and SI (1.6 ± 0.7vs. 1.9 ± 0.9 mm, p = 0.01) direction, but not in the ML direction (p = 0.279). The AP setup error increased 0.06 mm (y = 0.055x + 2.927, x: percentage of weight loss/PTV, y: AP displacement) per one percent increase in weight loss normalized to PTV.
  
  
  CONCLUSIONS
  Patients with weight loss > 5% and smaller PTVs, possibly because of small body frame or neck girth, were more likely to have increased setup errors in the AP direction.
authors.affiliation: Department of Radiation Oncology, City of Hope National Medical Center, Duarte, California, United States of America.
authors.forename: Wei-Hsien
authors.initials: WH
authors.lastname: Hou
authors.affiliation: Division of Radiation Oncology, Departments of Oncology, National Taiwan University College of Medicine and Hospital, Taipei, Taiwan, Province of China.
authors.forename: Chun-Wei
authors.initials: CW
authors.lastname: Wang
authors.affiliation: Department of Radiation Oncology, City of Hope National Medical Center, Duarte, California, United States of America.
authors.forename: Chiao-Ling
authors.initials: CL
authors.lastname: Tsai
authors.affiliation: Department of Radiation Oncology, City of Hope National Medical Center, Duarte, California, United States of America.
authors.forename: Feng-Ming
authors.initials: FM
authors.lastname: Hsu
authors.affiliation: Division of Radiation Oncology, Departments of Oncology, National Taiwan University College of Medicine and Hospital, Taipei, Taiwan, Province of China; Graduate Institutes of Oncology, National Taiwan University College of Medicine, Taipei, Taiwan, Province of China; Graduate Institute of Clinical Medicine, National Taiwan University College of Medicine, Taipei, Taiwan, Province of China.
authors.forename: Jason Chia-Hsien
authors.initials: JC
authors.lastname: Cheng
keywords: intensity-modulated radiotherapy; nasopharyngeal cancer; setup errors