Mitotic count of fine needle aspiration material of gastrointestinal stromal tumours of the stomach underestimates actual mitotic count

Introduction A mitotic count is required for histological grading in resections of gastrointestinal stromal tumours (GISTs). However, no consensus on the utility of mitotic count in fine needle aspiration (FNA) GIST material currently exists. This study examines the relationship between mitotic counts of FNAs and subsequent resections of GISTs of the stomach. Materials and methods We identified 39 cases of GISTs of the stomach diagnosed via FNA at our institution between January 1, 2014, and December 31, 2019, with subsequent resection. We noted if rapid on-site evaluation (ROSE) was performed. Cell block (CB) material from FNAs was analysed for total area, percentage containing neoplastic cells, and number of mitoses. We compared the mitotic counts in CBs and subsequent resections with t tests. Results ROSE was performed in 82% of cases and called adequate every time. Mean values for total CB area, neoplastic material percentage, and area of neoplastic cells were 54.7 mm(2) (range 1-986), 45% (range 10%-90%), and 19.2 mm(2), respectively; 27 cases (69%) had greater than 50 high powered fields of GIST material in the CB. Mean numbers of mitoses per 5 mm(2) were 0.38 (range 0-11) for CBs versus 5.92 (range 0-70) for resections (P < 0.05). Conclusion At our institution, ROSE adequacy of spindle cell lesions focuses on diagnosing GIST rather than obtaining adequate material for histological grading. Mitotic figures were statistically lower in FNA CB material than subsequent resections, and using mitotic counts from CB material may underestimate the histological grade of GISTs of the stomach.