Gastrointestinal Stromal Tumors (GIST)

See Small Bowel Neoplasms for the general small-bowel-tumor overview; GISTs span the benign-to-malignant spectrum and are covered together here rather than split across the benign/ malignant neoplasm pages, matching how the source chapter treats them. Content below is from a textbook reference chapter (Sabiston Ch50 - Infectious Enteritis & Small Bowel Neoplasms), not primary literature.

Origin and epidemiology

GISTs make up 20% of all soft tissue sarcomas occurring throughout the GI tract, most prevalent in the stomach (60%) and jejunum/ileum (30%), rarely the duodenum (5%). Stromal tumors arise from the interstitial cell of Cajal, an intestinal pacemaker cell of mesodermal descent. Median age at diagnosis is 65 years, similar rates in males and females. In the small bowel specifically, GISTs are the most common benign lesions that produce symptoms (bleeding, obstruction) even though adenomas are more common in autopsy series — see Small Bowel Neoplasms. Malignant GISTs constitute about 20% of malignant small bowel neoplasms; malignant GISTs are larger than 5 cm in 80% of patients at diagnosis.

GISTs can be malignant tumors, and nearly 50% of patients are found to have metastatic disease, most commonly to the liver.

Clinical manifestations

Symptoms of GIST include abdominal pain, fullness, bowel obstruction, or tumor hemorrhage resulting in anemia, melena, or hematemesis (small bowel GISTs are associated with GI bleeding more than other small bowel tumor types — see Small Bowel Neoplasms). Malignant potential is reportedly between 35% and 55%.

Diagnosis

Workup is often initiated with a CT scan; MRI may provide more information for tumors in the rectum or duodenum. An endoscopic core biopsy with immunohistochemical staining for KIT (95%) and anoctamin-1 (98%) confirms the diagnosis. More than 95% of stromal tumors express CD117, the KIT proto-oncogene protein (a transmembrane receptor for the stem cell growth factor); 70–90% express CD34, the human progenitor cell antigen. These tumors infrequently stain positive for actin (20–30%), S100 (2–4%), and desmin (2–4%). In gross appearance, stromal tumors are firm, gray-white lesions with a whorled appearance on cut surface; microscopically well-differentiated smooth muscle cells. These tumors grow intramurally and cause obstruction, or alternatively grow intramurally and extramurally, sometimes achieving considerable size and eventually outgrowing their blood supply, resulting in bleeding manifestations.

Radiologic criteria for unresectability: infiltration of the celiac trunk, superior mesenteric artery, or portal vein. Lymphadenectomy is unnecessary given the low frequency of lymph node metastasis. Small GISTs (<2 cm) found incidentally in surgical specimens do not require further treatment.

Prognostic factors

The most useful indicators of survival and metastasis risk include size of the tumor at presentation, mitotic index, and evidence of tumor invasion into the lamina propria.

Mitotic index / risk stratification: GIST malignancy risk and prognosis are stratified based on the number of mitoses per high-power field (hpf) and tumor size. Mitotic index is classified as low (<5 mitoses/50 hpf) or high (>5 mitoses/50 hpf). While benign tumors generally show a low mitotic index, the size of the tumor must also be considered. Tumors larger than 5 cm, regardless of mitotic index, have higher rates of metastasis and recurrence, while those with a high mitotic index have a higher risk of metastasis and recurrence regardless of size. Higher-risk GIST lesions may require adjuvant therapy after resection.

Treatment

Surgical resection is necessary for appropriate treatment and represents one of the first breakthroughs in signal transduction manipulation. The treatment regimen is based on localized vs. metastatic disease (Fig. 50.37 in the source gives the full current algorithm).

• Localized GIST: surgical management includes complete resection with extreme care to avoid rupture of the tumor capsule, which results in relapse in 100% of cases if it occurs. If capsule rupture occurs, these patients should receive adjuvant therapy regardless of the extent of the tumor before surgery. It is advisable to perform an en bloc resection, including adjacent organs, to prevent tumor capsule rupture. A laparoscopic approach in patients with large tumors is strongly discouraged.
  • Expected R0 resection: proceed to surgery. If not, assess for a KIT-sensitive mutation; if sensitive, neoadjuvant imatinib for 6–12 months, then reassess for R0/R1 resection feasibility. If R0/R1 feasible, surgery; if not, follow the advanced/metastatic GIST algorithm.
  • Post-resection, sensitive-mutation patients are risk-stratified: low risk → disease surveillance; high risk → adjuvant imatinib for 36 months.
• Advanced/metastatic GIST: treatment depends on whether the tumor carries a sensitive c-KIT mutation. Exon 11 mutations: imatinib 400 mg, escalate to continue imatinib 6–12 months if responding, or reassess for surgery/ablation of residual/progressing disease if limited progression, or move to sunitinib if no response/no limited progression. Exon 9 mutations: imatinib 800 mg with a similar response pathway. No sensitive mutation: imatinib or sunitinib via clinical trials. Sunitinib nonresponders progress to regorafenib, then to TKI rechallenge/clinical trials if disease continues to progress.

Before the development of tyrosine kinase inhibitors, adjuvant strategies for GIST were lacking, and recurrence rates after resection were as high as 70%. However, the development of imatinib mesylate (Gleevec) significantly altered previous treatment strategies. Imatinib mesylate is a tyrosine kinase inhibitor that blocks the unregulated mutant c-kit tyrosine kinase and inhibits the BCR-ABL and PDGF tyrosine kinases. Multiple randomized trials have confirmed its efficacy in the treatment of GIST. Current guidelines suggest patients with high-risk disease should receive 3 years of adjuvant imatinib treatment; it is not recommended for low-risk patients after an R0 resection. Neoadjuvant imatinib should be considered for patients requiring extensive surgery, to allow tumor shrinkage prior to resection.

Relapse-risk assessment for a primary GIST is critical, providing prognostic information as well as estimating potential benefits of medical therapy. Several available risk stratification systems include the National Institute of Health GIST consensus criteria, the American Forces Institute of Pathology criteria, the Joensuu risk criteria, and prognostic nomograms. In addition to staging information, several mutations carry prognostic implications — deletions affecting exon 11, codon 557/558 of the c-kit gene, and D842V PDGFRα mutations carry a higher recurrence risk within the first 3–4 years after surgery. Adjuvant imatinib is not recommended in patients with D842V PDGFRα mutations, given their known resistance to this agent.

Medical therapy for advanced/metastatic disease (detail)

Imatinib is first-line treatment for unresectable and metastatic GISTs with characteristic tumor biology. Genotyping is standard of care for advanced/metastatic disease. Standard-dose therapy (400 mg daily) is recommended, as no survival advantage is offered by dose escalation unless the patient has an exon 9 mutation — a European trial found patients with exon 9 mutations had a dose-dependent decrease in progression risk, so imatinib 400 mg twice daily should be given in this select group.

New molecular targeted therapies for genetic-mutation GISTs are an active area of study:

• Dasatinib (oral TKI of c-kit, PDGFR, ABL, and Src) showed significant activity (by CT response) in imatinib- and sunitinib-refractory GISTs in a phase 2 trial, though it did not meet the predefined 6-month progression-free survival rate of 30%. In vitro studies suggest dasatinib may be particularly useful for the D842V PDGFRα mutation subset.
• Regorafenib (second-generation TKI targeting c-kit, RET, BRAF, VEGFR, PDGFR, and fibroblast growth factor receptor) — FDA-approved for advanced GISTs after failure of either imatinib or sunitinib.
• Nilotinib (second-generation TKI active in chronic myeloid leukemia, inhibitory on c-kit and PDGF) — phase 3 trials showed minimal differences vs. imatinib or sunitinib.
• Sorafenib (VEGF, c-kit, PDGFR, and BRAF inhibitor) — effective in imatinib- and sunitinib-resistant tumors.
• Imatinib + doxorubicin combination has shown some benefit in wild-type GISTs.

Open items / gaps

• No primary literature yet in the wiki independently appraising imatinib RCTs or the risk stratification systems listed — currently only the textbook’s narrative summary.
• The ESMO-EURACAN algorithm (Fig. 50.37) is summarized in condensed form above; refer back to the source chapter figure for the full branching logic if precise treatment sequencing is needed clinically.