Underestimation of Subserosal Invasion in Early Gastric Cancer‐Like Advanced Gastric Carcinoma With Focal Enteroblastic Differentiation: A Case Report

ABSTRACT

Early gastric cancer (EGC)‐like advanced gastric cancer (AGC) is a rare entity that mimics superficial disease despite deep invasion. We report a case of EGC‐like AGC with focal enteroblastic differentiation in which subserosal invasion was underestimated because of its superficially appearing endoscopic features. A subtle submucosal tumor (SMT)‐like change was recognized only retrospectively after clinicopathological correlation. Endoscopy revealed a depressed 0‐IIc–appearing lesion with fold convergence toward a single point, leading to a diagnosis of UL1; however, no mound‐like elevation was observed. Retrospective review identified a subtle SMT‐like protrusion on the anterior wall, slightly apart from the main depression. Endoscopic submucosal dissection (ESD) was performed for presumed EGC. During ESD, a whitish cord‐like structure extending from the muscularis propria was identified at the corresponding site. The ESD specimen showed submucosal invasion with a positive vertical margin, and additional surgical resection revealed subserosal invasion. Histopathological examination of the surgical specimen demonstrated tubular adenocarcinoma (tub2>por1/tub1) with focal enteroblastic differentiation and subserosal invasion (pT3 [SS]) without lymph node metastasis but with venous invasion (V1c). Immunohistochemistry showed CK7 positivity, CK20 negativity, and partial expression of CDX2, glypican‐3, and SALL4, whereas AFP was negative. Clinicopathological correlation showed that the retrospectively recognized SMT‐like change corresponded to the deepest invasive component. This case highlights the difficulty of preoperative depth assessment in EGC‐like AGC. Careful integration of endoscopic, intraoperative, and clinicopathological findings may help avoid underestimation of tumor depth.