title: Repeat CT-scan assessment of lymph node motion in locally advanced cervical cancer patients.
abstract: OBJECTIVE
  In cervical cancer patients the nodal clinical target volume (CTV, defined using the major pelvic blood vessels and enlarged lymph nodes) is assumed to move synchronously with the bony anatomy. The aim of this study was to verify this assumption by investigating the motion of the major pelvic blood vessels and enlarged lymph nodes visible in CT scans.
  
  
  METHODS
  For 13 patients treated in prone position, four variable bladder-filling CT scans per patient, acquired at planning and after 40 Gy, were selected from an available dataset of 9-10 CT scans. The bladder, rectum, and the nodal-vessels structure containing the iliac vessels and all visible enlarged nodes were delineated in each selected CT scan. Two online patient setup correction protocols were simulated. The first corrected bony anatomy translations and the second corrected translations and rotations. The efficacy of each correction was calculated as the overlap between the nodal-vessels structure in the reference and repeat CT scans. The motion magnitude between delineated structures was quantified using nonrigid registration.
  
  
  RESULTS
  Translational corrections resulted in an average overlap of 58 ± 13% and in a range of motion between 9.9 and 27.3 mm. Translational and rotational corrections significantly improved the overlap (64 ± 13%, p value = 0.007) and moderately reduced the range of motion to 7.6-23.8 mm (p value = 0.03). Bladder filling changes significantly correlated with the nodal-vessels motion (p < 0.001).
  
  
  CONCLUSIONS
  The motion of the nodal-vessels was large, nonrigid, patient-specific, and only moderately synchronous with the bony anatomy. This study highlights the need for caution when reducing the CTV-to-PTV (PTV planning target volume) margin of the nodal CTV for highly conformal radiation techniques.
authors.affiliation: Department of Radiation Oncology, Erasmus Medical Center Cancer Institute, P.O. Box 5201, 3008 AE, Rotterdam, The Netherlands, lbondar@xs4all.nl.
authors.forename: Luiza
authors.initials: L
authors.lastname: Bondar
authors.forename: Laura
authors.initials: L
authors.lastname: Velema
authors.forename: Jan Willem
authors.initials: JW
authors.lastname: Mens
authors.forename: Ellen
authors.initials: E
authors.lastname: Zwijnenburg
authors.forename: Ben
authors.initials: B
authors.lastname: Heijmen
authors.forename: Mischa
authors.initials: M
authors.lastname: Hoogeman
mesh_terms: D000465:Algorithms; D005260:Female; D006801:Humans; D008198:Lymph Nodes / Q000000981:diagnostic imaging*; D008207:Lymphatic Metastasis; D009038:Motion; D011856:Radiographic Image Enhancement / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods*; D002583:Uterine Cervical Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy