title: Tumour ADC measurements in rectal cancer: effect of ROI methods on ADC values and interobserver variability.
abstract: OBJECTIVE
  To assess the influence of region of interest (ROI) size and positioning on tumour ADC measurements and interobserver variability in patients with locally advanced rectal cancer (LARC).
  
  
  METHODS
  Forty-six LARC patients were retrospectively included. Patients underwent MRI including DWI (b0,500,1000) before and 6-8 weeks after chemoradiation (CRT). Two readers measured mean tumour ADCs (pre- and post-CRT) according to three ROI protocols: whole-volume, single-slice or small solid samples. The three protocols were compared for differences in ADC, SD and interobserver variability (measured as the intraclass correlation coefficient; ICC).
  
  
  RESULTS
  ICC for the whole-volume ROIs was excellent (0.91) pre-CRT versus good (0.66) post-CRT. ICCs were 0.53 and 0.42 for the single-slice ROIs versus 0.60 and 0.65 for the sample ROIs. Pre-CRT ADCs for the sample ROIs were significantly lower than for the whole-volume or single-slice ROIs. Post-CRT there were no significant differences between the whole-volume ROIs and the single-slice or sample ROIs, respectively. The SDs for the whole-volume and single-slice ROIs were significantly larger than for the sample ROIs.
  
  
  CONCLUSIONS
  ROI size and positioning have a considerable influence on tumour ADC values and interobserver variability. Interobserver variability is worse after CRT. ADCs obtained from the whole tumour volume provide the most reproducible results. Key Points • ROI size and positioning influence tumour ADC measurements in rectal cancer • ROI size and positioning influence interobserver variability of tumour ADC measurements • ADC measurements of the whole tumour volume provide the most reproducible results • Tumour ADC measurements are more reproducible before, rather than after, chemoradiation treatment • Variations caused by ROI size and positioning should be taken into account when using ADC as a biomarker for tumour response.
authors.affiliation: Department of Radiology, Maastricht University Medical Center, P.O. Box 5800, 6202 AZ, Maastricht, The Netherlands.
authors.forename: Doenja M J
authors.initials: DM
authors.lastname: Lambregts
authors.forename: Geerard L
authors.initials: GL
authors.lastname: Beets
authors.forename: Monique
authors.initials: M
authors.lastname: Maas
authors.forename: Luís
authors.initials: L
authors.lastname: Curvo-Semedo
authors.forename: Alfons G H
authors.initials: AG
authors.lastname: Kessels
authors.forename: Thomas
authors.initials: T
authors.lastname: Thywissen
authors.forename: Regina G H
authors.initials: RG
authors.lastname: Beets-Tan
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D038524:Diffusion Magnetic Resonance Imaging; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D060787:Neoplasm Grading; D015588:Observer Variation; D012004:Rectal Neoplasms / Q000175:diagnosis* / Q000453:epidemiology / Q000473:pathology; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D047368:Tumor Burden