title: An analysis of respiratory induced kidney motion on four-dimensional computed tomography and its implications for stereotactic kidney radiotherapy.
abstract: OBJECTIVE
  Stereotactic ablative body radiotherapy (SABR) is an emerging treatment modality for primary renal cell carcinoma. To account for respiratory-induced target motion, an internal target volume (ITV) concept is often used in treatment planning of SABR. The purpose of this study is to assess patterns of kidney motion and investigate potential surrogates of kidney displacement with the view of ITV verification during treatment.
  
  
  METHODS
  Datasets from 71 consecutive patients with free breathing four-dimensional computed tomography (4DCT) planning scans were included in this study. The displacement of the left and right hemi-diaphragm, liver dome and abdominal wall were measured and tested for correlation with the displacement of the both kidneys and patient breathing frequency.
  
  
  RESULTS
  Nine patients were excluded due to severe banding artifact. Of 62 evaluable patients, the median age was 68 years, with 41 male patients and 21 female patients. The mean (range) of the maximum, minimum and average breathing frequency throughout the 4DCTs were 20.1 (11-38), 15.1 (9-24) and 17.3 (9-27.5) breaths per minute, respectively. The mean (interquartile range) displacement of the left and right kidneys was 0.74 cm (0.45-0.98 cm) and 0.75 cm (0.49-0.97) respectively. The amplitude of liver-dome motion was correlated with right kidney displacement (r=0.52, p<0.001), but not with left kidney displacement (p=0.796). There was a statistically significant correlation between the magnitude of right kidney displacement and that of abdominal displacement (r=0.36, p=0.004), but not the left kidney (r=0.24, p=0.056). Hemi-diaphragm displacements were correlated with kidney displacements respectively, with a weaker correlation for the left kidney/left diaphragm (r=0.45, [95% CI 0.22 to 0.63], p=<0.001) than for the right kidney/right diaphragm (r=0.57, [95% CI 0.37 to 0.72], p=<0.001).
  
  
  CONCLUSIONS
  For the majority of patients, maximal left and right kidney displacement is subcentimeter in magnitude. The magnitude of kidney motion cannot be reliably estimated from the diaphragmatic, liver dome or abdominal wall surrogates. One explanation may be that the kidneys are not uniformly in contact with the surrogates investigated in this study. Further investigation is required before surrogates of kidney displacement are used for clinical SABR delivery.
authors.affiliation: Department of Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Australia. shankar.siva@petermac.org.
authors.forename: Shankar
authors.initials: S
authors.lastname: Siva
authors.forename: Daniel
authors.initials: D
authors.lastname: Pham
authors.forename: Suki
authors.initials: S
authors.lastname: Gill
authors.forename: Mathias
authors.initials: M
authors.lastname: Bressel
authors.forename: Kim
authors.initials: K
authors.lastname: Dang
authors.forename: Thomas
authors.initials: T
authors.lastname: Devereux
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
authors.forename: Farshad
authors.initials: F
authors.lastname: Foroudi
mesh_terms: D000368:Aged; D016477:Artifacts; D002292:Carcinoma, Renal Cell / Q000532:radiotherapy; D005260:Female; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D007668:Kidney / Q000000981:diagnostic imaging / Q000473:pathology*; D007680:Kidney Neoplasms / Q000532:radiotherapy; D008099:Liver / Q000000981:diagnostic imaging; D008297:Male; D008875:Middle Aged; D009068:Movement; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012119:Respiration