Adenocarcinoma arising in gastric inverted hyperplastic polyp: a case report and review of the literature.

A 54-year-old man, found to have a submucosal tumor in the stomach by double contrast roentgenography, endoscopy, and endoscopic ultrasonography, underwent a laparoscopic partial gastrectomy. The pathological examination revealed that the lesion, measuring 45 mm x 35 mm, was an inverted hyperplastic polyp (IHP) located in the submucosal layer and consisting mostly of columnar cells resembling foveolar epithelium and pyloric gland cells. Notably, adenocarcinoma with adjacent dysplasia was observed in the submucosal glands. Transition from hyperplasia to dysplasia and from dysplasia to adenocarcinoma was noted. The adenocarcinoma component was intensely and diffusely positive for p53 overexpression, while the dysplasia component showed only weak and focal positivity, suggesting a role of p53 mutation in the dysplasia-carcinoma sequence. Gastric IHP is very rare, and only 31 cases (in 29 patients) have been reported. Five of these IHPs coexisted with gastric adenocarcinomas, which had all developed separately from the IHP lesions. Therefore, this is the first case of adenocarcinoma arising within gastric IHP itself.