16 Chapter 2 Abstract Objectives To investigate uniformity and pitfalls in structured radiological staging of rectal cancer. Methods Twenty-one radiologists (12 countries) staged 75 rectal cancers on MRI using a structured reporting template. Interobserver agreement (IOA) was calculated as the percentage agreement between readers (categorical variables) and Krippendorff’s alpha (continuous variables). Agreement with an expert consensus served as a surrogate standard of reference to estimate diagnostic accuracy. Polychoric correlation coefficients were used to assess correlations between diagnostic confidence and accuracy (=agreement with expert consensus). Results Uniformity to diagnose high-risk (≥cT3ab) versus low-risk (≤cT3cd) cT-stage, cN0 versus cN+, lateral nodes and tumour deposits, MRF and sphincter involvement, and solid versus mucinous tumours was high with IOA >80% in the majority of cases (and >80% agreement with expert consensus). Results for assessing extramural vascular invasion, cT-stage (cT1-2/cT3/cT4a/cT4b), cN-stage (cN0/N1/N2), relation to the peritoneal reflection, extent of sphincter involvement (internal/intersphincteric/external) and morphology (solid/annular/semi-annular) were considerably poorer. IOA was high (α=0.72-0.84) for tumour height/length and extramural invasion depth, but low for tumour-MRF distance and number of (suspicious) nodes (α=0.05-0.55). There was a significant positive correlation between diagnostic confidence and accuracy (=agreement with expert consensus) (p<0.001-p=0.003). Conclusions - Several staging items lacked sufficient reproducibility. - Results for cT- and N-staging improved when using a dichotomized stratification - There was a significant correlation between diagnostic confidence and accuracy (=agreement with expert consensus). Introduction MRI is the main diagnostic technique for local tumour staging in rectal cancer. Primary goals of MRI are to establish the presence of key risk factors associated with local recurrence to determine the need for neoadjuvant treatment, and to assess the local invasion of tumours into surrounding organs and structures to help guide the surgical
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