35 years old female with peranal bleeding

History
A 35 years old female with known type 1 neurofibromatosis presented with lower gastrointestinal bleeding. At upper and lower endoscopy the source of the bleeding could not be identified.

Imaging findings
Unenhanced (A), arterial phase (B), and portal venous phase (C) CT scans show a well-defined mass with heterogenous enhancement at the outer aspect of the small intestinal wall in the left lower quadrant (red circle in C). In the coronary reformat (D) it can be seen that there is pedunculate attachment (arrow) of the mass (arrow heads) to the intestinal wall.

Diagnosis
Gastrointestinal stroma tumor (GIST) of the small intestine at histology. Mitotic count of less than 5 per 50 high power fields.

Discussion
GISTs are mesenchymal neoplasms that arise either from the gastrointestinal tract or from other intraabdominal soft tissue. They probably originate from the interstitial cells of Cajal, which are located in the myenteric plexus and are pacemaker cells for gut peristaltic contractions. GISTs are most common in the stomach (60%) and the small intestine (30%) and less frequent in the colon and rectum (5%) and the oesophagus (<1%). GISTs found elsewhere within the abdominal cavity are referred to as extragastrointestinal or E-GISTs (<5% of all GISTs). The most frequent symptoms are bleeding into the bowel or abdominal cavity, anemia, and abdominal pain.

Generally GISTs occur sporadically, but some arise in the setting of specific tumor syndromes. A subset of patients with type 1 neurofibromatosis will develop GISTs, mainly in the small intestine.

The malignant potential of GISTs ranges from small lesions with a benign behaviour to aggressive sarcomas. About 40% of GISTs that are localised at the time of detection give rise to metastases, and 10 to 20% of patients with GISTs present with overt metastases. Metastases have a predilection to the liver, omentum, peritoneum, and other intra-abdominal sites, whereas metastases outside the abdomen are uncommon.

The most important prognostic factors for GIST recurrence after surgery are high tumor mitotic rate (mitotic count greater 5 per 50 high power fields), tumor size, tumor site, and tumor rupture before or at surgery (figure 1). Non-gastric GISTs and E-GISTs have a higher risk of recurrence compared with gastric GISTs. Most GISTs have activating mutations in the KIT or PDGFRA oncogene, which make them susceptible for treatment with the tyrosine kinase inhibitor imatinib.

Figure 1. Criteria for risk stratification of GIST recurrence after surgery.

Figure 1. Criteria for risk stratification of GIST recurrence after surgery.

Diagnostic imaging typically shows an exophytic mass outside the organ of origin that is usually arising from the organ periphery and sometimes is pedunculated. GISTs generally have well-defined margins. Heterogeneous enhancement with central necrosis or hemorrhage is common. Aneurysmal dilatation of small bowel lumen may occur due to cavitation and apparent luminal enlargement.

CT is the standard imaging method in patients with GISTs, because therapy response assessment consists not only of changes in lesion size but also of changes in lesion density. Lesion density in Hounsfield units (HU) should be assessed in the portal venous phase. In the widely accepted Choi criteria for GIST response assessment a 15% decrease of lesion density reflects response on CT images irrespective of changes in lesion size (figure 2).

Figure 2. GIST response criteria (Choi criteria)

Figure 2. GIST response criteria (Choi criteria)

Recommended reading
Kalkmann et al.
Consensus Report on the Radiological Management of Patients with Gastrointestinal Stromal Tumours (Gist): Recommendations of the German Gist Imaging Working Group
Cancer Imaging (2012) 12:126

Joensuu et al.
Gastrointestinal Stromal Tumour
Lancet (2013) 382:973

Choi et al.
Correlation of Computed Tomography and Positron Emission Tomography in Patients with Metastatic Gastrointestinal Stromal Tumor Treated at a Single Institution with Imatinib Mesylate: Proposal of New Computed Tomography Response Criteria
J Clin Oncol (2007) 25:1753

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