Osseous Metaplasia: An Unusual and Poorly Understood Finding in Gastric Polyps

Abstract

Abstract Osseous metaplasia (OM) is an uncommon finding in gastrointestinal tract biopsies. Metaplastic bone formation has been predominantly reported in the lower GI tract in a background of benign and malignant neoplasms, as well as nonneoplastic inflammatory conditions. OM in the stomach is extremely rare, with less than 10 cases reported in the English literature to date. We present two cases of gastric OM encountered at one institution among more than 10,000 stomach polyps examined since 1994. The first is a 46-year-old female with spastic quadriplegia, peptic ulcer disease, and a jejunal feeding tube who had multiple gastric hyperplastic polyps removed over a 13-year period. The most recent polypectomy revealed a 3-cm gastric adenoma with benign bone formation in the stroma. The second case is of a 67-year-old asymptomatic male with a remote history of intestinal metaplasia and atrophy of the stomach who underwent upper endoscopy and removal of a 1-cm polyp. Sections consisted of hyperplastic foveolar epithelium with focal ossification. Both polyps showed acute and chronic inflammation and stromal fibrosis, and they lacked Helicobacter infection. Mechanisms driving gastrointestinal OM have been proposed since the first reported cases in the early 1900s. The most widely accepted theory is that osteogenic factors released by the neoplastic or inflammatory tissue induce undifferentiated stromal mesenchymal cells or fibroblasts to undergo transformation to osteoblasts. However, with gastrointestinal biopsies being one of the most common specimens, the picture remains incomplete as only very rare cases show bone formation, warranting further research. Although gastrointestinal OM appears to be clinically insignificant, long-term studies have yet to be performed. Nevertheless, it is crucial for pathologists to be aware of this phenomenon in order to avoid misdiagnosing OM as a primary mesenchymal neoplasm or bone invasion in cases of OM arising in the background of malignancy.