[A Case Report of a Giant GIST Markedly Responsive to Neoadjuvant Chemotherapy with Imatinib Mesylate].

Male in his 50s complaining of abdominal pain was referred to our hospital. Abdominal CT scan showed a giant tumor which had diameter of approximately 50 mm in lower rectum. A biopsy specimen was positive for CD34 and c-kit. Based on these findings, it was diagnosed as gastrointestinal stromal tumor(GIST). We treated the patient with neoadjuvant therapy using imatinib mesylate(IM)to reduce the tumor size and to avoid the extensive surgery. The patient started to take IM at a daily dose of 400 mg. After 3 months, CT and MRI revealed that the tumor size decreased(40% reduction). We performed the robot assisted intersphincteric resection(ISR). Although it has been 28 months since the surgery, there are no obvious signs of recurrence. A patient diagnosed with giant GIST could avoid an extensive surgery due to neoadjuvant therapy with IM.