[615] Correlation of Ki67 Labelling Index in FNA and Biopsy of GIST with Ki67 and Mitotic Rate in Resection Specimens

Jennifer A Collins, William S Twaddell. University of Maryland Medical Center, Baltimore, MD

Background: Predicting the behavior of gastrointestinal stromal tumors (GISTs) is difficult and dependant on many different factors. Among these, the mitotic rate is particularly important; but even in tumors showing a relatively high mitotic rate this may be as low as 6 mitoses per 50 high-power fields (HPFs). Often, the diagnosis of GIST is made prior to resection or other treatment via biopsy or fine-needle aspiration (FNA). Given the small size of such samples, the 50 HPFs needed to assess for mitoses may not be present, or may not be representative of the final specimen. Immunostaining for Ki67, a protein indicating cellular proliferation, is utilized to calculate proliferation index. Therefore, staining for Ki67 may be useful for assessing proliferative rate in small GIST specimens obtained prior to resection.
Design: 19 cases were identified by a retrospective review in which both a resection specimen and a pre-resection were available: 18/19 were from the stomach, 1/19 from the duodenum with 13/19 from cytological specimens and 6/19 from biopsies. The mitotic rate was determined by 2 independent reviewers and the mean calculated. Immunohistochemical staining for Ki-67 was performed on all specimens and quantitated by computer-assisted image analysis using the Aperio system. The Ki-67 index of the cytology or biopsy specimen was correlated to the Ki-67 index and mitotic count of the final resected specimen.
Results: The Ki-67 index for the resection specimen was dichotomized based on the mitotic count of 5/HPF. For ≤5/50hpf (n=11) the range was from 0.5-3.76 and for >5/50hpf (n=8) the range was from 5-25. Correlation between mitotic count and ki-67 index on resection is r2=0.94 with a p-value<0.0001. The mean Ki-67 count for mitotic counts ≤5 is 1.8 (1.04-2.47 95% CI), the mean Ki-67 count for mitotic rates >5 is 9.3 (2.37-16.2 95% CI). Correlation of Ki-67 index between cytological/biopsy specimen and resection specimen is r2= 0.33 (p-value<0.001) but with exclusion of inflamed biopsy specimens (in which staining of inflammatory cells may inflate the proliferative index) correlation is r2= 0.73 (p-value <0.0001).
Conclusions: The importance of mitotic rate in risk stratification for GISTs is well-accepted and is incorporated into the cancer protocols used by the College of American Pathologists (CAP). In the context of our limited study it appears that the utilization of Ki67 in biopsies and cytological specimens has a good correlation to the final specimen.
Category: Gastrointestinal

Wednesday, March 6, 2013 1:00 PM

Poster Session VI # 81, Wednesday Afternoon


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