A Study of Regorafenib Combined With Envafolimab for Metastatic Gastrointestinal Stromal Tumors With Kit Gene Exon 17 Mutation That Failed Standard Treatment

Inclusion Criteria: * Age ≥ 18 years, no gender restriction; * Pathologically confirmed gastrointestinal stromal tumor (GIST); * At least one measurable target lesion according to mRECIST v1.1 criteria (non-lymph node lesion with a long axis ≥ 1.0 cm or long axis ≥ 2 slide thicknesses); imaging assessment within 14 days before the first dose; * Progression or intolerance after treatment with imatinib, sunitinib, regorafenib, or ripretinib; * Genetic testing includes primary or secondary KIT exon 17 mutation; * Adequate organ and bone marrow function, defined as follows: Hematology: Absolute neutrophil count (ANC) ≥ 1.5 × 10\^9/L; platelet count (PLT) ≥ 75 × 10\^9/L; hemoglobin (HGB) ≥ 9.0 g/dL. No use of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), red blood cell transfusion, or platelet transfusion within 14 days before testing; Liver and kidney function: For patients without liver metastasis, total bilirubin (TBIL) ≤ 1.5 × upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 × ULN. For patients with liver metastasis: TBIL ≤ 1.5 × ULN; ALT and AST ≤ 5 × ULN. Kidney function: serum creatinine (Scr) ≤ 1.5 × ULN; Adequate coagulation function, defined as international normalized ratio (INR) ≤ 1.5 or prothrombin time (PT) ≤ 1.5 × ULN; if the patient is on anticoagulant therapy, PT should be within the intended range of the anticoagulant; * Provide 15 paraffin-embedde…