We reviewed the clinical data and pathological characteristics of the four cases(see  Table 1 ). All of the four patients were men and they were aged 58-69 years. The clinical symptoms were not specific. Three cases were discovered during a health check-up (space-occupying lesion in the stomach), and one case manifested as gastric discomfort accompanied by an elevated serum CA19-9 level. The disease manifested endoscopically as SMTs ( Figures 1A. B ), with the lesion size ranging from 1.3 to 3.5 cm. Three lesions were located in the upper third of the stomach and one in the lower third of the stomach. No GC diagnosis was obtained after multiple gastroscopic biopsies. Three patients underwent local mass resection ( Figures 1C, D ), followed by radical gastrectomy for GC ( Figure 1E ), one underwent endoscopic submucosal dissection (ESD).
The local mass resection specimens of the four cases, including the ESD specimens, were dissected along the largest lesion diameter perpendicular to the mucosal surface, to examine the lesions. Tumours were mainly located in the submucosa. The size of the tumours was in the 1.3-3.5 cm range, and they exhibited a greyish-white cut surface and unclear boundaries, and were mucinous in certain cases (75%, 3/4). For the three patients who underwent radical gastrectomy after local mass resection, specimens were collected using the routine procedure for radical gastrectomy specimen extraction, and the residual masses in the specimens were not observed by the naked eye.
Three cases of mucinous adenocarcinoma were found, among which two were well-differentiated mucinous adenocarcinomas and one was a poorly differentiated mucinous adenocarcinoma. Another case had gastric carcinoma with lymphoid stroma (GCLS, i.e., lymphoepithelioma-like carcinoma). Under the microscope, the SMT specimen of the patient with a poorly differentiated mucinous adenocarcinoma exhibited a nodular distribution with copious extracellular mucin ( Figure 2A ). Tumour cells were epithelium-like, possessed a large nucleus and distinct nucleoli, and floated within mucin pools in small nest-like clusters ( Figure 2B ). Necrotic nuclear fragments were also observed. The microscopic examination of the well-differentiated mucinous adenocarcinoma specimens revealed the presence of dense hyperplasia of the submucosal glands, a large nucleus, distinct nucleoli, obvious nuclear division, and local glandular fusion. This was locally accompanied by the formation of mucin pools ( Figures 2C, D ). Certain regions exhibited glandular ulcerations with spill over of mucin into the fibrous mesenchyme, which was accompanied by hyperplasia of the fibrous granulation tissue. Histological examination of the SMT specimens of case 4 showed the presence of a nodular structure and a lymphocyte-rich mesenchyme, accompanied by lymphoid follicle formation ( Figure 2E ). At high magnification, the tumour cells exhibited a symplastic appearance with a dichroic cytoplasm, a large nucleus, distinct nucleoli, and an irregular nest- and sheet-like arrangement ( Figure 2F ). The involvement of the muscularis propria was found in certain cases (50%, 2/4). The tumor cells were found to have migrated to the epithelial cells of the mucosal layer in only two cases. Among the three patients who underwent extended resection, one exhibited a small amount of local residual mucinous adenocarcinoma in the submucosa and metastasis in one out of 18 lymph nodes. In the other two cases, no metastasis was found in lymph nodes dissected.
The tumour cells of the four cases showed differing expression levels of the gastric mucins MUC1 (4/4), MUC5AC (2/4), MUC6 (2/4), CEA (4/4), CKpan (4/4)( Figure 2G ) and CK7 (3/4). The intestinal markers MUC2, CDX2 and CK20 were expressed in case 1, and EBER was expressed in the tumour cells of case 4 ( Figure 2H ). Ki-67 expression indicated that the tumour cell proliferation index was in the 20%-70% range. The tumor cells of four cases did not express CD10 and SATB2 proteins, no mutant expression of P53, and no deletion of mismatch repair proteins MLH1, PMS2, MSH2 and MSH6.