The GIST decreased from its initial size of 13.5 x 8.7 cm in November 2008 to 9.0 x 6.0 cm in January 2009. The primary tumour continued to decrease in size from 6.3 x 3.7 cm in June 2009 to 5.2 x 3.5 cm in November 2009. Figure 4 CT scan of the abdomen following treatment with imatinib mesylate revealing a reduction of GIST (top arrow). The colon mass is now visible (bottom arrow)

The CT scan in November 2009 revealed the presence of a colonic mass with mesenteric lymphadenopathy. The presence of the newly identified mass was confirmed on colonoscopy, Inhibitors,research,lifescience,medical which revealed the presence of an intraluminal mass at 80 cm from the anal verge. Biopsy of this lesion revealed an invasive, moderately differentiated adenocarcinoma of colonic origin. After discussion at tumor board, a Inhibitors,research,lifescience,medical decision was made to resect the primary colonic mass as well as the primary GIST. In December 2009, the patient underwent a left hemicolectomy in addition to resection of the

primary GIST, which originated in the small bowel. The pathology Inhibitors,research,lifescience,medical of the colonic mass revealed a moderately differentiated adenocarcinoma with 7 out 12 lymph nodes involved. The small bowel pathology revealed a spindle cell lesion consistent with a GIST, which was positive for CD117 and CD34. The Ki67 stain showed LEE011 order positivity in less than 1% of tumour cells. The mitotic count was less than 1 per 50 High Power Fields (HPF). The tumour showed large hypocellular areas of hyalinization, an area of Inhibitors,research,lifescience,medical necrosis, and several areas of hemorrhage as well as a focal hemangiopericytoma-like pattern, consistent with treatment (imatinib mesylate) effect. Of note, the laboratory findings did not include a preoperative CEA, however, a CEA level was drawn shortly after the surgery, measuring 2.5 ug/L. She subsequently received 12 cycles of modified FOLFOX-6 chemotherapy while remaining on imatinib for her metastatic

GIST. She did not experience any unexpected toxicity from either the imatinib or chemotherapy and remains well with continued regression Inhibitors,research,lifescience,medical of her liver metastasis (GIST). Case 2 A 61-year-old Caucasian gentleman presented with a change in bowel habits and rectal bleeding in March 2009. He reported no associated anorexia or weight loss. Colonoscopy and biopsy revealed an adenocarcinoma at the splenic flexure. A staging CT scan also revealed a few subcentimeter lymph nodes and a 5 cm mass at the gastrohepatic ligament also Thymidine kinase suspected to be an enlarged metastatic lymph node (Fig 5). Figure 5 CT scan demonstrating a mass later confirmed to be a primary gastric GIST In May 2009, at the time of surgery, the gastrohepatic mass was resected. Once confirmed on a frozen section to be a spindle cell tumour consistent with a GIST, a partial gastrectomy was performed. During the same operation, the patient also underwent a left hemicolectomy.