Reactive Nodular Fibrous Pseudotumor Presenting as Gastric Outlet Obstruction in a Young Adult Male Patient: A Case Report and Review of the Medical Literature.

Mesenchymal proliferations involving the gastrointestinal tract are uncommon and often unexpected upon initial clinical presentation. Here, we are reporting a case of a 20-year-old male patient presenting to the hospital with abdominal pain and vomiting. Upon evaluation, the patient was initially discharged home on antacid therapy. The symptoms progressed in intensity, and a subsequent CT scan was remarkable for marked gastric expansion and a prepyloric mass. The macroscopic examination of the resected lesion revealed a 5.6 cm multilobular fibrotic mass involving the gastric wall. Microscopic analysis was notable for a spindle-cell lesion with admixed lymphoplasmacytic inflammation, patchy necrosis, and calcifications. The immunohistochemical workup was suggestive of a reactive nodular fibrous pseudotumor. This case is notable for several aspects, including the acute clinical presentation, intralesional calcifications, abundant IgG4-positive plasma cells, and lack of CD117 immunohistochemical expression. The peculiarities of the case and a review of the medical literature will be presented.