Ki‐67 index assessment on <scp>FNA</scp> specimens of gastrointestinal stromal tumor: Correlation with mitotic rate and potential predictive value for risk stratification

Risk assessment of gastrointestinal stromal tumor (GIST) is challenging on cytology specimens. This study aims to determine whether Ki-67 index evaluated on fine-needle aspiration (FNA) specimens can correlate with the mitotic rate of GIST in surgical specimens and provide further risk assessment.Cases with cell blocks containing adequate tumor cells and surgical resections were included. Ki-67 immunostain was retrospectively performed on cell block sections, and Ki-67 index was calculated on the "hot spot" areas.This study included 50 GIST cases from stomach (n = 45; 90%), duodenum (n = 4; 8%), and distal esophagus (n = 1; 2%). The tumor size ranged from 1.5 cm to 21 cm (mean, 5.4 cm). Based on the mitotic count, 37 GISTs (74%) had low mitotic rate (LMR) and 13 GISTs (26%) had high mitotic rate (HMR). The spindle cell, epithelioid, and mixed types accounted for 60%, 14%, and 26% of GIST, respectively. Ki-67 index counted on cell block sections correlated well with mitotic count evaluated in surgical specimens (r = 0.8031). Mean Ki-67 index was higher in HMR than LMR groups (3.5% vs. 1%, p < .001). The receiver operating characteristic curve using Ki-67 index to predict mitotic rate was further analyzed, and area under the curve was 0.839. Using a cutoff of 2.5% yielded a sensitivity of 70% at 92% specificity.This study demonstrates good correlations between Ki-67 index and mitotic count or risk stratification, suggesting that Ki-67 index evaluated on cytology specimens may offer a promising approach to preoperatively predict the mitotic rate and risk of GIST.