Gastrointestinal stromal tumors are mesenchymal neoplasms of the gastrointestinal tract, arising from the muscular layer. Revent immunohistochemical studies have shown that up to 94% of GISTs express 117, and 60–70% of GISTs stain for CD 34. GISTs account for 1% of all malignant tumors of gastrointestinal tract; they arise from stomach in the 40–60% of cases whereas they account only for 3% of gastric malignant neoplasms. GISTs can occur at any age but onset most commonly in the sixth and the seventh decades of life.

We report the case of a 69-year-old male patient admitted to our surgical department with weakness, progressive abdominal straining and weight loss of 5 kg in 6 months.

Physical examination showed a voluminous mass palpable in the upper- to mid abdomen. The computed tomography (CT) revealed an enormous solid lesion, heterogeneous, polilobate, hypervascular, arising from peritoneal sheet and mesentery, dislocating small bowel and causing hydronephrosis I grade (see Figure 1).

Surgery was performed in October 2008. At laparotomy appeared an enormous solid mass, hypervascular, originating from the anterior wall of the stomach of 15 × 20 cm of diameter. The neoplasm was completely isolated and a partial gastrectomy Billroth 2 was performed along with a partial resection of omentum and the radical excision of the tumor (see Figure 2). The mass weighed 7 kg. The postoperative course was uneventful and the patient was discharged ten days after surgery.

The final histological findings confirmed the diagnosis of malignant neoplasm pT2 with fused cells compatible with fibrosarcoma poorly differentiated. The immunohistochemistry of the fused cell showed no positivity for CD 34, actin, ema andcytokeratins. The proliferative index was 7 mitoses per 10 HPF.

As reported in literature the GIST described in our case presented aspecific clinical finding and a not clear origin at CT scanning. In our case the high mitotic index (>5 mitoses per 10 HPF) and the tumor size greater than 5 cm manifest an aggressive biological behavior associated with a poor prognosis. In this case anyway the tumor doesn't result positive to CD 117 or CD 34 though presenting fused cells with mesenchimal phenotype.