Immunohistochemical testing of GISTs using CD117 markers: the UK NEQAS ICC & ISH external quality assessment data show significant differences in the performance of methods in regular use.

Background: CD117 (c-KIT) is a tyrosine kinase receptor protein, mutations in which are important in the tumourigenesis of gastrointestinal stromal tumour (GIST). Immunohistochemical detection of CD117 is the primary identifying feature in its diagnosis. Introduction: UK NEQAS ICC & ISH conducts an EQA programme for CD117 expression in GIST. Between May-2020 and May-2025, 20 runs of this EQA were undertaken. The archived data was analysed to look for evidence regarding the quality of testing and associations between primary antibodies and stain quality. Discussion: Total submissions were 2,656 (mean per run = 132.8; range = 125-140). The number of submissions awarded a quality score indicating at least acceptable stain quality was 2,642 (99.5%). The total number of submissions that failed was 14 (mean proportion per run = 0.5%; range = 0.0%-2.4%); those obtaining a borderline score totalled 158 (mean proportion = 5.9%; range = 0.7%-10.9%); those obtaining an acceptable score totalled 394 (mean proportion = 14.8%; range = 4.3-24.4%); and those obtaining a good/excellent score totalled 2,090 (mean proportion = 78.7%; range = 57.7%-94.2%). There was a strong trend for the proportion of submissions obtaining a good/excellent quality score to increase over time. Four primary antibodies were used: Clone 9.7 (102 submissions, 3.9%), YR145 (263 submissions, 10.0%), EP10 supplied by Leica Biosystems (EP10(LB), 415 submissions, 15.8%), EP10 supplied by Roche Diagnostics (EP10(RD), 505 submissions, 19.2%), and a polyclonal antiserum supplied by Agilent Dako (1,263 submissions, 48.1%). A total of 76 submissions (2.9%) that used other primaries or suppliers were excluded from analysis. Conclusion: Demonstration of CD117 overexpression in GIST was undertaken to a very high quality standard in the majority of laboratories. For users of Dako Agilent supplied automation the highest quality staining was produced by using the polyclonal antiserum from the same supplier. Among users of Leica Biosystems and Roche Diagnostics automation the best staining was produced by using primary sourced from the same supplier that provided their automation. It is recommended that Clone 9.7 (Roche Diagnostics) should not be used as the results it produced were statistically significantly inferior to those of all other antibodies.