Proliferation scores in canine anal sac adenocarcinomas: Ki67 global score is superior to Ki67 hotspot indices and mitotic count for prognosis

Canine anal sac adenocarcinoma (ASAC) is an aggressive malignancy with high metastatic potential. Histologic and proliferation parameters such as mitotic count and Ki67 scores have limited prognostic value according to the published literature. Using pathologist-supervised digital image analysis methods with the image analysis software QuPath, we analyzed 58 cases of ASAC to evaluate mitotic count (MC) and Ki67 indices, explore relationships between different Ki67 indices [semi-automatic Ki67 digital hotspot score (Ki67-saHDS), Ki67 global digital score (Ki67-GDS), and fully automatic Ki67 digital hotspot score (Ki67-faHDS)] and MC, and to verify which method carries the most significant prognostic value. The MC did not impact median tumor-related survival (TRS) time. Although high correlation coefficients were observed between the 3 Ki67 scores, Ki67-GDS had more prognostic relevance than hotspot-based scores (Ki67-saHDS and Ki67-faHDS). Dogs with Ki67-GDS ≥ 26 had significantly shorter survival times (175, days 95%, confidence interval (95% CI) = 123-540) compared to dogs with Ki67-GDS< 26 (median survival time (MST) 650 days, 95% CI = 503->1579). No association was observed between TRS and Ki67-faHDS or Ki67-saHDS. On multivariate analysis, anisokaryosis and Ki67-GDS, but not tumor size, lymphovascular invasion, or MC, were independent prognostic markers for survival. These results demonstrate the advantage of Ki67 GDS over hotspot-based scores; however, these data need to be validated in a larger cohort of cases before clinical implementation.