Variation in breast cancer grading in 1,636 resections assessed using control charts and in silico kappa

OBJECTIVE: Assess interpretative variation in Nottingham grading using control charts (CCs) and in silico kappa (ISK). METHODS: In house invasive breast cancer cases (2011–2019) at two institutions with a synoptic report were extracted. Pathologist interpretative rates (PIRs) were calculated and normed for Nottingham grade (G) and its components (tubular score (TS), nuclear score (NS), mitotic score (MS)) for pathologists interpreting >35 cases. ISKs were calculated using the ordered mutually exclusive category assumption (OMECA) and maximal categorical overlap assumption (MCOA). RESULTS: The study period included 1,994 resections. Ten pathologists each assessed 38–441 cases and together saw 1,636; these were further analyzed. The PIR medians (normed ranges) were: G1:24%(18–27%), G2:53%(43–56%) and G3:26%(19–33%). The MCOA ISK and the number of statistical outliers (p< 0.05/p< 0.001) to the group median interpretive rate (GMIR) for the ten pathologists was G1: 0.82(2/0 of 10), G2: 0.76(1/1), G3: 0.71(3/1), TS1: 0.79(1/0), TS2: 0.63(5/1), TS3: 0.66(5/1), NS1: 0.37(5/4), NS2: 0.60(4/3), NS3: 0.59(4/4), MS1: 0.78(3/1), MS2: 0.78(3/1), MS3: 0.77(2/0). The OMECA ISK was 0.62, 0.49, 0.69 and 0.71 for TS, NS, MS and G. CONCLUSIONS: The nuclear score has the most outliers. NS1 appears to be inconsistently used. ISK mirrors trends in conventional kappa studies. CCs and ISK allow insight into interpretive variation and may be essential for the next generation in quality.