Extremely well-differentiated gastric adenocarcinoma arising in gastric adenomyoma

Gastric adenomyoma, a subtype of pancreatic heterotopia, is a rare mural lesion typically located within the antrum or pylorus consisting of well-formed glands lined by a single layer of bland epithelium embedded within dense smooth muscle bundles. Like most forms of pancreatic heterotopia, gastric adenomyomas are usually incidental findings, but may result in obstructive symptoms, especially if situated within the pyloric channel. Malignant degeneration is exceedingly rare, with existing reports lacking detailed morphologic descriptions. Herein, we discuss the case of a 62-year old female who presented to our institution with two months of early satiety and bloating. Imaging revealed a partially cystic mass within the pyloric wall associated with eccentric gastric wall thickening. Endoscopic fine needle aspiration cytology revealed glands embedded in smooth muscle. Distal gastrectomy ultimately revealed an extremely well-differentiated adenocarcinoma arising in a pre-existing adenomyoma of the pyloric channel. In this report, an extensive review of this rare entity is provided, including detailed morphologic and immunohistochemical findings of our case, differential diagnostic considerations, pitfalls of gastric mural lesions that contain epithelium, and a comprehensive literature review.