gastrointestinal stromal tumor of the abdominal soft tissue

Gastrointestinal stromal tumor (GIST) typically occurs in the gastrointestinal (GI) tract, and expresses KIT protein that is associated with KIT or platelet-derived growth factor receptor-α (PDGFRA) gene mutation.

Extragastrointestinal stromal tumors (EGISTs) are a minor subset of GIST that occurs in the soft tissue outside the GI tract, and in very rare cases, these tumors can be KIT negative.

Location

 omentum
 mesentery
 retroperitoneum
 pelvic cavity
 not otherwise specified regions of the abdominal cavity.

Synopsis

 size from 4 to 33 cm (median, 15 cm) in maximum diameter
 relatively low mitotic counts (median, 3.5 per 50 high-power fields)
 Morphologically, most cases are of epithelioid cell or mixed epithelioid and spindle cell type, accompanied by variable amounts of myxoid stroma.

Immunochemistry

 CD34 (80%)
 protein kinase C (PKC) θ (90%)
 discovered on GIST-1 (DOG1) (90%)
 negative for KIT (0%).

Molecular biology

The majority of the examined cases (78%) have PDGFRA mutations in exon 12 or exon 18.

Rare cases (11%) have a mutation in KIT exon 11.

Very rare cases have no mutation in either KIT or PDGFRA.

KIT-negative EGISTs can be characterized by preferential omental origin, epithelioid cell type, low mitotic activity, and mutation of the PDGFRA gene, and these features are similar to those of KIT-negative gastric GISTs.

As KIT-negative EGISTs should be considered to be a potential abdominal soft tissue neoplasm, immunohistochemical staining panel and molecular analysis are necessary not only to confirm the diagnosis but also to determine the therapeutic strategy.

References

 KIT-Negative Gastrointestinal Stromal Tumor of the Abdominal Soft Tissue: A Clinicopathologic and Genetic Study of 10 Cases. Yamamoto H, Kojima A, Nagata S, Tomita Y, Takahashi S, Oda Y. Am J Surg Pathol. 2011 Sep;35(9):1287-95. PMID: 21836495