Gastrointestinal stromal tumors

GIST 101 - The Gut's Odd Lump

• What: Most common mesenchymal tumor of the GI tract; arises from the Interstitial Cells of Cajal (gut pacemaker cells).
• Mutation: Gain-of-function mutation in the KIT gene (a receptor tyrosine kinase) is the primary driver.
• Location: 60% Stomach > 30% Small Intestine > Esophagus/Colon/Rectum.
• Presents: Often asymptomatic. Can cause GI bleeding (melena, hematemesis), abdominal pain, or a palpable mass.
• Histo: Look for uniform spindle cells.
• Diagnosis: Immunohistochemistry (IHC) is key-tumors are positive for KIT (CD117).

⭐ The presence of the KIT mutation is not just diagnostic; it's the target for specific chemotherapy (tyrosine kinase inhibitors like Imatinib).

Pathogenesis - KIT's Wild Ride

• Origin: GISTs arise from the Interstitial Cells of Cajal (ICCs), the specialized pacemaker cells regulating GI motility.
• Primary Driver: The vast majority are driven by activating (gain-of-function) mutations in the KIT proto-oncogene, a receptor tyrosine kinase.

• A smaller subset involves mutations in the related PDGFRA gene.
• 📌 KIT is the "key" that's stuck, turning the ignition on permanently for cell growth.

⭐ Exam Favorite: The specific KIT exon mutation predicts imatinib sensitivity. Exon 11 mutations (~70% of KIT mutations) are most common and show the best response, while exon 9 mutations often require higher doses.

Diagnosis & Staging - The Stromal Reveal

• Initial Workup:
  • Endoscopy/EUS reveals a submucosal mass, sometimes with ulceration.
  • Contrast-enhanced CT defines tumor size, location, and metastatic spread (liver, peritoneum).
• Definitive Diagnosis: Biopsy & IHC
  • EUS-guided fine-needle aspiration (FNA) is the preferred method. ⚠️ Percutaneous biopsy risks peritoneal seeding.
  • Immunohistochemistry (IHC) is key:
    • KIT (CD117): Positive in ~95% of cases.
    • DOG1: Highly specific marker.
    • CD34: Positive in ~70%.

• Risk Stratification (NIH Criteria):
  • Mitotic Rate: High risk if >5 mitoses/50 HPF.
  • Tumor Size: High risk if >5 cm.
  • Location: Gastric GISTs have a better prognosis than those in the small intestine.

⭐ GISTs with a PDGFRA exon 18 D842V mutation are primarily resistant to imatinib therapy.

Management - The Imatinib Era

• Primary Therapy: Imatinib mesylate, a tyrosine kinase inhibitor (TKI).
  • Targets gain-of-function mutations in c-KIT (CD117) and PDGFRA.
  • Standard dose: 400 mg daily.
• Indications:
  • Unresectable, recurrent, or metastatic GIST.
  • Adjuvant therapy for resected GISTs with a high risk of recurrence.
• Resistance:
  • Primary: Rare, e.g., PDGFRA D842V mutation.
  • Secondary (acquired): Sunitinib (2nd line), Regorafenib (3rd line).

⭐ Exam Favorite: Over 70% of GISTs have a KIT gene mutation. The most common, in exon 11, predicts a favorable response to imatinib therapy.

High‑Yield Points - ⚡ Biggest Takeaways

• GISTs are the most common mesenchymal tumors of the GI tract, originating from the interstitial cells of Cajal.
• Most are driven by gain-of-function mutations in the KIT gene (CD117), a receptor tyrosine kinase.
• Histology typically shows bundles of spindle cells.
• The stomach is the most frequent site (>50%), followed by the small intestine.
• Imatinib, a tyrosine kinase inhibitor, is the cornerstone of targeted therapy.
• Prognosis depends on tumor size, mitotic rate, and location.