Answer: Inflammatory fibroid polyp

Histology: This biopsy specimen from the antrum of the stomach at low magnification shows a poorly circumscribed fibroinflammatory lesion centered in the lamina propria which splays apart fibers of the muscularis mucosae to involve the submucosa. There is overlying mucosal erosion. At higher power, the lesion is variably cellular and composed of plump to elongated spindle cells in a myxoid stromal background which contains a mixed inflammatory infiltrate with prominent eosinophils. There is focal germinal center formation. The spindled stromal cells and eosinophils form characteristic concentric rings around large and small blood vessels when seen in cross section.

Discussion: Inflammatory fibroid polyp (IFP), described by Vanek in 1949 is a mesenchymal reactive tumifactive process involving the GI tract. The exact nature and etiology remain elusive and controversial but many issues relating to their diagnosis and differential diagnosis have been clarified. The stomach, antrum in particular, is the most common site of involvement but they also occur in the small and large intestine and very rarely in the esophagus. It is more common in females and can occur at all ages.

The IFP especially in the stomach is poorly circumscribed, infiltrative and centered in the submucosa but may impinge on the mucosa and tunica muscularis. The ileal IFP tends to be a bit more circumscribed and polypoid. Complications include ulceration, bleeding and intussusception.
This non-neoplastic tumor is composed of proliferating spindle cells in a loose myxoid vascular stroma admixed with assorted inflammatory cells such as plasma cells, lymphocytes, histiocytes and a generous sprinkling of eosinophils. In the majority of the lesions, the spindle cells show a loose concentric arrangement (onion skinning) around blood vessels and glands. In a small subset of these lesions this arrangement is not seen. This and other observations have lead to the speculation that there may be two different lesions included under the term inflammatory fibroid polyp. The spindle cells are positive for CD34 (negative in those tumors lacking concentric onion skinning) in the majority of cases, as is vimentin. Histiocytic markers are also positive in some cases. The lesions are negative for muscle and neural markers. CD117 (c-kit) is consistently negative. Genetically no substitution, deletion, insertion occur in c-kit exon 11 or c-kit exon 9.
A diffuse positivity for fascin and focal positivity for CD35 in 10 cases has suggested a dendritic cell phenotypic differentiation.

Distinction should be made of IFP from other reactive, neoplastic and quasineoplastic spindle cell mass lesions such as granulation tissue, eosinophilic gastroenteritis, gastrointestinal stromal tumor, inflammatory pseudotumors (inflammatory myofibroblastic tumors also known as plasma cell granuloma), benign fibroblastic polyps, solitary fibrous tumor, hemangioendothelioma and hemangiopericytoma. This differential diagnosis may be particularly challenging in small endoscopic mucosal biopsies from the GI tract.