GIST encompass a broad continuum of histologic patterns, ranging from bland-appearing tumors with very low mitotic activity (often previously designated leiomyomas) to very aggressive-appearing patterns (previously often called leiomyosarcomas). They may originate from interstitial cells of Cajal (ICC) or their stem cell-like precursors, although this is not certain.[15,16]
The most commonly used marker for GIST is the CD117 antigen, a marker expressed by ICC. Approximately 95% of GISTs are positive for the CD117 antigen, an epitope of the KIT receptor tyrosine kinase.[2,9] However, CD117 immunohistochemistry is not specific for GIST, as weak reactivity occurs with other mesenchymal neoplasms; accordingly, morphologic examination and the use of other immunostains in difficult cases are indispensible. In addition, false-positive CD117 staining can occur if antigen retrieval techniques are used in the pathology laboratory to enhance marker expression. Because of a relatively broad morphologic spectrum, the differential diagnosis of GIST includes several mesenchymal, neural, and neuroendocrine neoplasms that occur in the abdomen including the following:
- Malignant peripheral-nerve sheath tumor.
- Solitary fibrous tumor.
- Inflammatory myofibroblastic tumor.
- Synovial sarcoma.
- Neuroendocrine tumors (carcinoid and islet cell).
- Gastric glomus tumor.
- Malignant mesothelioma.
- Sarcomatoid carcinoma.
Approximately 85% of GIST contain oncogenic mutations in one of two receptor tyrosine kinases: KIT or PDGFRA (platelet-derived growth factor receptor alpha).[2,10] Constitutive activation of either of these receptor tyrosine kinases plays a central role in the pathogenesis of GIST.[15,19] Wild-type tumors, with no detectable KIT or PDGFRA mutations, account for 12% to 15% of all GIST. Fewer than 5% of GIST occur in the setting of syndromic diseases, such as neurofibromatosis type 1 (NF1), Carney triad syndrome, and other familial diseases.[2,20,21,22] The correct identification of GIST is very important because of the availability of specific, molecular-targeted therapy with KIT/PDGFRA tyrosine kinase inhibitors (TKI) such as imatinib mesylate or, in the case of imatinib-resistant GIST, sunitinib malate.[1,10,17]
Risk Assessment and Prognosis
At the time of clinical presentation, the prognosis appears to be influenced by genetic events other than kinase mutations, although a particular kinase mutation may help to define the initial clinical course of a GIST. Based on retrospective studies from time periods that predated the clinical use of kinase inhibitors, current recommendations for assessing the risk of progression for a newly diagnosed primary GIST rely on three parameters (see Table 1):[2,23,24,25,26]
- Mitotic index (mitoses per 50 high-power fields).
- Tumor size.
- Tumor location.