This chapter discusses the case of a 58-year-old woman who presented with pain and a mass in the right iliac fossa. Ultrasound and CT imaging showed a 19 cm mass with compression of the right ureter and liver metastases. Percutaneous biopsy of the mass was undertaken. This showed features of a spindle cell neoplasm. Immunohistochemical staining was positive for cKIT, indicating a diagnosis of gastrointestinal stromal tumor (GIST). At operation, there was no evidence of liver disease. Radical resection was undertaken, with en bloc resection of the tumor mass, adjacent sigmoid colon, lower 3 cm of ureter and right ovary and fallopian tube. A laparoscopic biopsy of the abdominal mass might have been oncologically preferable to a percutaneous biopsy in view of the potential for tumor seeding along the needle track. Following a percutaneous biopsy, ideally the track should be excised or incorporated within the surgical incision.