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PATHOBIOLOGY & DIAGNOSIS
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Gastrointestinal mesenchymal tumors (which include stromal tumors, leiomyomas, and schwannomas) derive from mesenchymal stem cells and have an epithelioid or spindle cell histologic pattern, resembling smooth muscle. The most common stromal tumors are gastrointestinal stromal tumors (“GISTs”), which originate from interstitial cells of Cajal. GISTs occur throughout the gastrointestinal tract, but most commonly in the stomach (60%) and small intestine (30%). Approximately 80% of GISTs have mutations in KIT, a receptor tyrosine kinase that binds stem cell factor, or a homologous tyrosine kinase, platelet-derived growth factor alpha that lead to constitutive kinase activation. Approximately 95% of stromal tumors stain positively for CD117 (part of the KIT protein). The presence of multifocal GISTs is associated with neurofibromatosis-1. Other mesenchymal tumors, such as leiomyomas, which derive from smooth muscle cells, generally stain negative for CD117. A percentage of the KIT wild type tumors have a mutation in PDGFRA, and only a minority of patients are wild type for both genes. Mesenchymal tumors may be discovered incidentally on imaging studies or endoscopy or may cause symptoms (most commonly bleeding, pain, or obstruction). At endoscopy, they appear as a submucosal mass that may have central umbilication or ulceration (eFigure 39–6). EUS with guided FNA biopsy is the optimal study for diagnosing gastric mesenchymal tumors and distinguishing them from other submucosal lesions. Percutaneous biopsy may confer risk of bleeding or intra-abdominal seeding. CT of the abdomen and pelvis with contrast, MRI, and PET imaging are useful in the diagnosis and staging. PET imaging also may be useful to monitor response to treatment.
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While almost all GISTs have malignant potential, the risk of developing metastasis is increased with tumor size greater than 2 cm, nongastric location, and mitotic index greater than 5 mitoses per 50 HPF. It is difficult to distinguish benign from malignant tumors by EUS appearance or by FNA. But, in general, lesions are more likely benign if they are smaller than 2 cm, have a smooth border, and have a homogeneous echo pattern on EUS. Resection settles the issue.
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A. Localized Treatment
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Surgery is recommended for all patients with tumors that are 2 cm or larger, increasing in size, have an EUS appearance suspicious for malignancy, or are symptomatic. The management of asymptomatic gastric lesions 2 cm or smaller in size depends on the EUS features. Tumors with high-risk EUS features can be surgically resected. If no high-risk features are noted, endoscopic surveillance can be performed. Because of the low but real long-term risk of malignancy, surgical resection should be considered in younger, otherwise healthy patients. However, other patients may be monitored with serial endoscopic ultrasonographic ...