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Pathologic Complete Response in a Large Gastric GIST: Using Molecular Markers to Achieve Maximal Response to Neoadjuvant Imatinib

Joshua B. Brown, Reetesh K. Pai, Melissa A. Burgess, Jennifer Chennat, and Amer H. Zureikat

Gastrointestinal stromal tumors (GISTs) represent 1% of alimentary tract neoplasms. Up to 90% of GISTs are driven by activating mutations in tyrosine kinase KIT or PDGFRα genes. Imatinib mesylate is a tyrosine kinase inhibitor that has recently been used in a neoadjuvant role for locally advanced GIST. Pathologic complete response (pCR) to imatinib, however, is rare and may be limited to patients with certain mutations. We report on a 71-year-old woman with a large advanced gastric GIST near the gastroesophageal junction initially involving the pancreas, spleen, adrenal, and aortic wall. The tumor harbored a KIT exon 11 deletion mutation in codon 558, which predicts a favorable response to imatinib. After 6 months of neoadjuvant imatinib therapy, the tumor was downstaged to allow partial gastric resection without the need for total gastrectomy reconstruction. The patient underwent partial gastrectomy, distal pancreatectomy, and splenectomy, and histologic examination showed a margin-negative resection with a near-pCR, with <5% viable tumor. Prolonged neoadjuvant therapy was undertaken based on the prognostic significance of a KIT exon 11 deletion mutation in codon 558, which facilitated an R0 resection while minimizing the surgical extent of the resection. A near-pCR of a large gastric GIST after neoadjuvant imatinib therapy remains a rare occurrence. Molecular testing should be undertaken before neoadjuvant therapy, because specific mutations can identify patients who will respond to imatinib and those likely to achieve significant downstaging and pCR.