[86] KIT-Negative Gastrointestinal Stromal Tumor of the Abdominal Soft Tissue: A Clinicopathological and Genetic Study of 10 Cases.

Hidetaka Yamamoto, Yoshinao Oda. Kyushu University, Fukuoka, Japan

Background: Gastrointestinal stromal tumor (GIST) typically occurs in the digestive tract and express KIT protein associated with KIT or platelate-derived growth factor receptor-alpha (PDGFRA) gene mutation. Minor subset of GISTs are immunohistochemically negative for KIT, or occur in the soft tissue outside the gastrointestinal tract, namely extragastrointestinal stromal tumor (EGIST). Furthermore, KIT-negative EGISTs are very rare.
Design: We examined the clinicopathological and molecular characteristics of 10 cases of KIT-negative EGIST by using immunohistochemical stain and gene mutation analysis.
Results: The tumors occurred in omentum (n=5), mesenterium (n=2), retroperitoneum (n=1), pelvic cavity (n=1) and not-otherwise-specified abdominal cavity (n=1). They ranged from 4 to 33 cm (median 15 cm) in maximum diameter with relatively low mitotic counts (median 3.5 per 50 high-power-fields). Morphologically, most cases were epithelioid cell (n=9) or mixed epithelioid and spindle cell (n=1) type, accompanied by variable amount of myxoid stroma. By immunohistochemical stain, the tumors were positive for CD34 (80%), PKC theta (90%) and DOG1 (90%), but negative for KIT (0%). The majority of cases (7/9 cases tested; 78%) had PDGFRA mutations in exon 12 (n=1) or exon 18 (n=6). One case (11%) had the mutation in KIT exon 11, and the remaining one had no mutation in both KIT and PDGFRA genes.
Conclusions: The morphologic, phenotypic and genotypic features of KIT-negative EGIST are similar to those of KIT-negative gastric GIST. Although the origin of EGIST is debatable (really soft tissue primary vs. GI primary with secondary extension to soft tissue and eventual loss of connection to GI wall), KIT-negative EGIST should be considered as a potential abdominal soft tissue neoplasm. Immunohistochemical stain and molecular analysis are necessary not only to confirm the diagnosis but also to determine the therapeutic strategy.
Category: Bone & Soft Tissue

Monday, February 28, 2011 1:00 PM

Poster Session II # 1, Monday Afternoon


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