Reactive nodular fibrous pseudotumor: a first report of gastric localization and clinicopathologic review.

Reactive nodular fibrous pseudotumor (RNFP) of the gastrointestinal tract is a distinct benign lesion, which could originate from a reactive proliferation of multipotential subserosal cells. This is the first case to be reported in the stomach. It was fortuitously discovered in a 60-year-old man with history of bulbar ulcer and gastritis. Gross examination revealed three lesions in the gastric wall and an adjacent lesion in the lesser omentum. Histologically, lesions were composed of a proliferation of spindle and stellate cells in a dense collagenic hyalinized background containing a mononuclear cell inflammatory infiltrate with numerous lymphoid aggregates and plasma cells with perivascular disposition. Immunohistochemistry showed staining for cytokeratins (AE1/AE3), vimentin and smooth muscle actin, without staining for the neurofilament and S100 proteins, synaptophysin, calretinin, CD117 (c-kit), CD34, desmin, caldesmon or anaplastic lymphoma kinase (ALK-1). Complete excision was performed, and no evidence of disease was found 4 months later. After analysing clinical, morphological and immunohistochemical features of this entity, the main differential diagnoses will be discussed, including calcifying fibrous pseudotumor, which shares morphological characteristics with RNFP, but which immunohistochemistry and the ultrastructural study suggest that it may be a result of another reactive process.