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

A 74-year-old man complaining of perianal pain was referred to our hospital. A contrast-enhanced computed tomography scan showed a giant mass measuring 10.3 cm between the rectum and the prostate. Magnetic resonance imaging showed contrast effect and decreased diffusion at the margins, with hemorrhage and necrosis inside the mass. We performed a transperineal needle biopsy, it was diagnosed as rectal gastrointestinal stromal tumor(GIST). Complete resection of the tumor was difficult, we started neoadjuvant chemotherapy with imatinib mesylate(400 mg/day)for reduced the diameter of the tumor. The tumor diameter(reduction rate)at 1, 3, and 5 months after treatment was 7.4 cm(28%), 7.0 cm(32%), and 6.2 cm(40%). We considered complete surgical resection possible and performed a robot-assisted rectal resection with no tumor damage. Pathological examination was consistent with a rectal GIST, and the resection margins were negative. We report a case of a giant rectal GIST markedly responsive to neoadjuvant chemotherapy with imatinib mesylate.