A 24 year old woman presented with a painless abdominal lump of six months’ duration. She had no history of colonic polyps. A mobile, non-tender, globular mass was felt in the umbilical region. A computed tomography scan showed a homogenous, non-enhancing mass, possibly arising in the small bowel mesentery.

The tumour was resected entirely with a loop of jejunum. The tumour measured 14 × 12 × 10 cm and was intimately related to the bowel wall. The cut surface was tan, whorled, and firm, without necrosis, cystic change, or haemorrhage. Microscopy showed loosely and haphazardly arranged spindle cells with bland, oval nuclei and minimal cytoplasm (figs 1 and 2). There were also plump spindle cells with tapering ends with oval, vesicular nuclei and moderate amounts of eosinophilic cytoplasm (fig 3).

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Figure 1

 Spindle cell neoplasm infiltrating the muscularis propria of the jejunum.

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Figure 2

 Bland spindle cells and many thin walled vessels forming the neoplasm.

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Figure 3

 Spindle cells with tapering ends and oval, vesicular nuclei.

There were many thin walled vessels of varying calibre (fig 2). There were no cells with epithelioid features, inflammatory cells, calcification, osseous metaplasia, necrosis, or mitoses. The tumour infiltrated the muscularis propria and had infiltrating margins. The tumour cells were negative with antibodies to CD117, S100, CD34, and smooth muscle actin (Dako, St Louis, Missouri, USA; dilutions of 1/200, 1/200, 1/50, and prediluted, respectively) and positive for desmin (Labvision, Freemont, California, USA; 1/200 dilution).

The patient has no evidence of disease 16 months after surgery.

The differential diagnosis of a bland spindle cell tumour involving the gastrointestinal tract and mesentery includes gastrointestinal stromal tumour (GIST), fibromatosis, and inflammatory myofibroblastic tumour.¹ Inflammatory myofibroblastic tumours are more common in children and are characterised by a dense inflammatory cell component among a myofibroblastic proliferation. GIST was considered an unlikely diagnosis because of the whorled appearance on cut surface; histological evaluation confirmed this. We made a diagnosis of fibromatosis of the jejunum and mesentery.

Intra-abdominal fibromatosis, in its classic presentation, as a mesenteric mass, does not pose a diagnostic problem because of its distinctive gross and microscopic features. However, when it presents primarily as an intestinal wall tumour, the diagnosis of GIST may seriously be considered. Importantly, as many as five of 13 cases of bowel wall fibromatosis in one series had been labelled initially as low grade sarcomas, whereas 13 of 25 in another study mimicked GIST.^2,3 Distinguishing between the two entities is important because of the different treatment protocols and biological behaviour. GISTs are malignant neoplasms that may be treated by Imanitib mesylate if they are unresectable or if there is a distant metastasis.⁴ They are soft, lobulated, and fleshy on cut surface. Epithelioid cells, skeinoid fibres, mitoses, and necrosis are common.^1–3 Fibromatosis is a low grade neoplasm that may recur but never metastasises. Recurrence is often related to incomplete excision. Tamoxifen has been used in the management of a recurrent or unresectable tumour.⁵ GISTs usually express CD117 and often CD34 also, whereas fibromatosis is always negative for CD34 and may or may not express CD117.^1–3 However, fibromatosis is essentially a haematoxylin and eosin diagnosis. The gross appearance—that is, a fibrous mass without necrosis or haemorrhage—gives a clue to the diagnosis.