Anorectum

Anorectal GISTs account for 9% of 64 GISTs, according to a recently published series.27 Unlike GISTs in other locations, a male predominance has been reported.14,52 Clinical presentation includes rectal pain, bleeding, and rectal mass. Anorec-tal GISTs present as eccentric mural masses that invade the rectal wall. The most common finding at CT is a focal well-circumscribed mural mass, which expands the rectal wall and

figure 30.7. Multiple GISTs in a 42-year-old man with history of neurofibromatosis (type I) and lower gastrointestinal bleeding. Axial CT of the lower abdomen in the arterial phase reveals multiple pedun-culated skin lesions (arrowhead) and within the abdomen (small arrows) compatible with neurofibromas. In addition, there is an enhancing soft tissue mass in the distal jejunum (arrow), which was proven to be a GIST at small bowel resection.

figure 30.7. Multiple GISTs in a 42-year-old man with history of neurofibromatosis (type I) and lower gastrointestinal bleeding. Axial CT of the lower abdomen in the arterial phase reveals multiple pedun-culated skin lesions (arrowhead) and within the abdomen (small arrows) compatible with neurofibromas. In addition, there is an enhancing soft tissue mass in the distal jejunum (arrow), which was proven to be a GIST at small bowel resection.

figure 30.8. Poorly differentiated high-grade duodenal sarcoma in a 61-year-old man. (A) Axial CT of the abdomen with oral and intravenous contrast reveals a soft tissue mass in the region of the pancreatic head (arrow), involving the descending portion of the duodenum. This mass could be mistaken for pancreatic neoplasm. (B) Axial CT of the abdomen at a level lower than (A) shows invasion of the right renal hilum (arrow) and the inferior vena cava (arrowhead). A ureteric stent is seen in place (small arrow). (C) Coronal volume-rendered image shows the epicenter of the lobulated mass along the descending portion of the duodenal mass, with surrounding mesenteric fat stranding (arrow) indicating peritumoral spread of disease. The pancreas is well visualized (arrowheads) and is normal.

figure 30.9. Malignant GIST in a 40-year-old woman presenting with rectal mass. (A) Axial noncontrast CT of the pelvis performed as part of positron emission tomography (PET)/CT reveals a large pelvic soft tissue mass displacing the air-filled rectum (arrow) and urinary bladder (arrowhead) anteriorly. (B) Axial noncontrast CT at a level lower than (A) reveals extension of the mass into the left ischiorectal fossa (arrow). (C) Coronal whole-body PET image reveals intense fluo-rodeoxyglucose (FDG) uptake indicating increased metabolic activity of the tumor (arrow), which is common in patients with untreated GISTs.

figure 30.9. Malignant GIST in a 40-year-old woman presenting with rectal mass. (A) Axial noncontrast CT of the pelvis performed as part of positron emission tomography (PET)/CT reveals a large pelvic soft tissue mass displacing the air-filled rectum (arrow) and urinary bladder (arrowhead) anteriorly. (B) Axial noncontrast CT at a level lower than (A) reveals extension of the mass into the left ischiorectal fossa (arrow). (C) Coronal whole-body PET image reveals intense fluo-rodeoxyglucose (FDG) uptake indicating increased metabolic activity of the tumor (arrow), which is common in patients with untreated GISTs.

may ulcerate (Figure 30.9). The mass may extend into the ischiorectal fossa and may invade surrounding pelvic organs, including the vagina, prostate, or urinary bladder. These findings may result in difficulty identifying the organ of origin on cross-sectional imaging. Similar to GISTs in other locations of the gastrointestinal tract, enhancement is heterogeneous with areas of low attenuation from hemorrhage or necrosis. Calcification and adenopathy are rare. GISTs in the anorectal region show malignant behavior even when small (less than 2 cm in maximum dimension) and have no more than five mitoses per 50 high-power fields. These tumors are also associated with significant mortality rates.53

Rectal adenocarcinoma, anal squamous cell carcinoma, leiomyoma, leiomyosarcoma, lymphoma, and malignant melanoma may have a CT appearance similar to anorectal GIST and are in the differential diagnosis. Leiomyosarcoma may have a dominant polypoid mass while adenocarcinoma may have an irregular margin, soft tissue invasion into ischiorectal fossa, and perirectal adenopathy.14 Primary anorectal lymphoma may be seen in patients with acquired immunodeficiency syndrome and may be associated with mucosal ulceration or perianal fistula. Other features that may be seen in primary anorectal lymphoma, include tumor heterogeneity, concentric wall thickening, intraluminal polypoid masses, and thickening of adjacent levator ani muscle.27

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