The proliferation of spindle cells merges with overlying mucosa. Lesions generally arise from the submucosa but can extend into the mucosa itself. A brisk eosinophilic infiltrate is appreciated, as well as admixed lymphocytes.
This is a loosely structured lesion. The spindle cells have oval or spindled nuclei with finely granular chromatin and indistinct nucleoli
Collagen is randomly distributed with lymphocytes and many eosinophils scattered about. The spindle cells have vesicular nuclei and there is some hyalinization of this tumor. Although difficult to see, there is a subtle concentric nature to the fibroblasts around the vessels (see thickened vessel on the upper right), so called onion-skinning. CD34 and smooth muscle actin are often positive in these lesions but they are uniformly CD117 (c-kit) negative.
Inflammatory fibroid polyps (IFP) are benign proliferation of spindle cells with blood vessels and an inflammatory infiltrate with prominent eosinophils. IFPs can affect any part of the GI tract but are most commonly seen in the stomach at the antrum. The recent identification of PDGFRA-activating mutations suggests strong evidence of clonal proliferation and a neoplastic nature of IFP (Daum).
Grossly, the mass is well-circumscribed, solitary, pedunculated or sessile. It is typically located in the submucosa and surface ulceration is quite common. Histologically, there is a proliferation of spindle cells in an edematous stroma. Sometimes, the proliferation centers on a blood vessels, forming concentration rings called "onion-skinning". There is usually a chronic inflammatory infiltrate heavy in eosinophils.
A note on inflammatory fibroid polyps versus inflammatory pseudotumors (inflammatory myofibroblastic tumor): These tumors are different! Whereas IFPs occur in predominately in the stomach and small intestine, whereas inflammatory pseudotumors (now renamed inflammatory myofibroblastic tumors) occur predominately in the lungs and soft tissues. The molecular aberrations are different as well, with inflammatory fibroid polyps demonstrating mutations in PDGFRA and inflammatory pseudotumors demonstrating aberrations in the anaplastic lymphoma kinase (ALK)-receptor tyrosine kinase gene (Daum, Coffin).
Although perhaps a bit simplistic, the inflammatory infiltrate in inflammatory fibroid polyps is rich in eosinophils, whereas the most prominent inflammatory cells in inflammatory pseudotumor are plasma cells (Fletcher).
Adults and children may be affected but most common in the older population. Usually are incidental findings, but occasionally, large polyps may result in gastric outlet obstruction (Cerwenka). 70% of IFPS, occur in the stomach with the remainder arising in the ileum, colon, jejunum, duodenum and eosphagus.
Benign lesion, cured by resection. Occasionally, large polyps may result in gastric outlet obstruction.
Cerwenka H, Bacher H, Mischinger HJ. Pyloric obstruction caused by prolapse of a hyperplastic gastric polyp. Hepatogastroenterology. 2002 Jul-Aug;49(46):958-60.
Coffin CM, Hornick JL, Fletcher CD. Inflammatory myofibroblastic tumor: comparison of clinicopathologic, histologic, and immunohistochemical features including ALK expression in atypical and aggressive cases. Am J Surg Pathol. 2007 Apr;31(4):509-20.
Daum O,et al. Comparison of morphological, immunohistochemical, and molecular genetic features of inflammatory fibroid polyps (Vanek's tumors). Virchows Arch. 2010 May;456(5):491-7.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 366, 1553.