Interactive digital slides with heat maps: a novel method to improve the reproducibility of Gleason grading

Our aims were to analyze reporting of Gleason pattern (GP) 3 and 4 prostate cancer with the ISUP 2005 Gleason grading and to collect consensus cases for standardization. We scanned 25 prostate biopsy cores diagnosed as Gleason score (GS) 6-7. Fifteen genitourinary pathologists graded the digital slides and circled GP 4 and 5 in a slide viewer. Grading difficulty was scored as 1-3. GP 4 components were classified as type 1 (cribriform), 2 (fused), or 3 (poorly formed glands). A GS of 5-6, 7 (3 + 4), 7 (4 + 3), and 8-9 was given in 29%, 41%, 19%, and 10% (mean GS 6.84, range 6.44-7.36). In 15 cases, at least 67% of observers agreed on GS groups (consensus cases). Mean interobserver weighted kappa for GS groups was 0.43. Mean difficulty scores in consensus and non-consensus cases were 1.44 and 1.66 (p = 0.003). Pattern 4 types 1, 2, and 3 were seen in 28%, 86%, and 67% of GP 4. All three coexisted in 16% (11% and 23% in consensus and non-consensus cases, p = 0.03). Average estimated and calculated %GP 4/5 were 29% and 16%. After individual review, the experts met to analyze diagnostic difficulties. Areas of GP 4 and 5 were displayed as heat maps, which were helpful for identifying contentious areas. A key problem was to agree on minimal criteria for small foci of GP 4. In summary, the detection threshold for GP 4 in NBX needs to be better defined. This set of consensus cases may be useful for standardization.