Gastrointestinal Stromal Tumor (GIST): KIT Mutations and Targeted Therapy — Overview, Diagnosis & Treatment Options | MyMedicPlus

Gastrointestinal stromal tumors (GISTs) are the most common primary mesenchymal tumors of the GI tract, arising from interstitial cells of Cajal or their precursors, which are the pacemaker cells of gut motility. Approximately 4,000-6,000 new US cases are diagnosed annually. Primary sites include the stomach (60%), small intestine (25-35%), colon and rectum (5%), and esophagus (1%). Approximately 80% harbor activating KIT mutations; 10% harbor PDGFRA mutations.

Activating somatic mutations in KIT (approximately 80%) or PDGFRA (approximately 10%) drive most GISTs by constitutively activating receptor tyrosine kinase signaling. Wild-type GISTs (without KIT or PDGFRA mutations) include SDH-deficient GISTs (pediatric and young adult, associated with Carney triad and Carney-Stratakis syndrome), NF1-associated GISTs, and BRAF V600E-mutant GISTs. Hereditary GIST syndrome results from germline KIT or PDGFRA mutations. Imatinib-sensitizing mutations in KIT exon 11 are the most common (approximately 65% of all GISTs).

Small GISTs are frequently asymptomatic and discovered incidentally during endoscopy or cross-sectional imaging. Larger tumors cause GI bleeding (hematemesis or melena) as the most common symptom, followed by abdominal pain, abdominal mass, early satiety, nausea, and dysphagia (for esophageal tumors). Acute abdomen from tumor rupture with hemoperitoneum is a rare but serious presentation. Constitutional symptoms (weight loss, fatigue) suggest advanced disease with liver or peritoneal metastases.

Contrast-enhanced CT is the imaging modality of choice, showing a well-circumscribed hypervascular heterogeneous mass with central necrosis in large tumors. Endoscopic ultrasound (EUS) with FNA is valuable for small submucosal tumors. KIT (CD117) immunostaining is positive in approximately 95% of GISTs; DOG1 (TMEM16A) is a sensitive and specific marker. KIT and PDGFRA mutational analysis by Sanger sequencing or NGS is mandatory before systemic therapy to determine imatinib dose and response prediction. Risk stratification (NIH/AFIP criteria) guides adjuvant therapy decisions.

Complete surgical resection (R0) without lymphadenectomy is the primary treatment for localized, resectable GIST. Adjuvant imatinib for 3 years (400 mg/day) is recommended for high-risk disease; adjuvant imatinib for 1 year may be considered for intermediate-risk. Neoadjuvant imatinib for 6-12 months can downsize large or borderline resectable tumors to allow R0 resection. Metastatic GIST: imatinib 400 mg/day (800 mg for KIT exon 9); sunitinib second-line; regorafenib third-line; ripretinib fourth-line; avapritinib for PDGFRA D842V-mutant disease.

Localized GIST: 5-year disease-specific survival approximately 90% for very low risk, dropping to approximately 50% for high-risk without adjuvant imatinib. Three years of adjuvant imatinib for high-risk GIST improved 5-year recurrence-free survival from 48% to 66% and improved 5-year OS from 81% to 92% (SSGXVIII trial). Metastatic GIST: median OS with imatinib is approximately 4-5 years. KIT exon 11 mutation confers best response to imatinib and longest survival. SDH-deficient GISTs progress slowly and rarely respond to imatinib.

No established prevention exists for sporadic GIST. Patients with NF1 syndrome should be informed of elevated GIST risk and undergo investigation of any GI symptoms. Families with hereditary GIST syndrome (germline KIT or PDGFRA mutations) should receive genetic counseling, with first-degree relatives offered germline testing. SDH-deficient GISTs in young patients warrant evaluation for Carney triad (GIST, paraganglioma, pulmonary chondroma) or Carney-Stratakis syndrome (SDHA/B/C/D germline mutation), requiring surveillance for associated tumors.