Gastric-Type Mixed Neoplastic and Non-Neoplastic Lesions in the Duodenal Bulb: A Case Supporting the Metaplasia-Neoplasia Sequence.

Gastric-type lesions in the duodenum, including pyloric gland adenoma and gastric foveolar metaplasia, have been increasingly recognized for their unique histogenesis and potential link through the metaplasia-neoplasia sequence. However, the coexistence of neoplastic and non-neoplastic gastric-type lesions within the same histological section has not been previously reported. Here, we present a case of a 73-year-old Japanese woman who underwent endoscopic submucosal dissection for a 34 × 20 mm elevated lesion in the duodenal bulb. Based on the preoperative biopsy results, pyloric gland adenoma was diagnosed; however, histopathological examination of the resected specimen revealed a far more complex picture. The main lesion consisted of two contiguous components: a hyperplastic polyp with gastric foveolar-type phenotype (Lesion I) and a pyloric gland adenoma mixed with gastric foveolar-type hyperplastic polyp (Lesion II). Importantly, the transitional zone between these components demonstrated histological continuity, with areas showing admixture of hyperplastic and adenomatous features within the same microscopic field. A separate hyperplastic polyp with gastric foveolar-type phenotype (Lesion III) was also identified, separated from Lesions I and II by intervening normal duodenal mucosa. All lesions shared a gastric-type mucin phenotype (MUC5AC-positive, CD10-negative), and extensive Brunner's gland hyperplasia was observed throughout the specimen. This case provides compelling morphological evidence for a histogenetic link between non-neoplastic gastric-type hyperplasia and pyloric gland adenoma, supporting the concept of a metaplasia-neoplasia sequence in the duodenum. Furthermore, the presence of an additional separate lesion with the same phenotype suggests a field change in the development of gastric-type lesions.