Validity and reproducibility of Ki-67 assessment in gastrointestinal stromal tumors and leiomyosarcomas

With the aim of standardizing Ki‐67 immunohistochemistry, we assessed interobserver and interlaboratory variability of the Ki‐67 labeling index and Ki‐67 score among eight general pathologists for 24 gastrointestinal stromal tumors (GISTs) and 12 leiomyosarcomas, which were predominantly of the gastrointestinal (GI) tract, mesentery and retroperitoneum, based on a review of a tissue microarrays subjected to immunohistochemistry with antibodies for Ki‐67. For Ki‐67 immunostaining of mesenchymal tumors of the GI tract, including GISTs, differences were seen in the scores given by regional hospitals. Conversely, for two categories of the Ki‐67 labeling index, namely