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

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.