ECP 2023 Abstracts

S205 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 metastases showing carcinosarcomatous features. Our objective is to highlight the characteristic histopathological and immunohistochemical findings of this aggressive tumour. Methods: The 83-year old woman was admitted due to acute epigastric pain. CT scan showed advanced gastric cancer with perigastric lymph node enlargement and CBD stones. The patient underwent subtotal gastrectomy with lymphadenectomy. On gross examination, a huge ulceroinfiltrative mass (9x6cm) was located in the antrum along greater curvature. The largest metastatic node measured up to 4.5cm in the diameter. Results: On microscopic examination, the cuboidal to columnar tumour cells have clear cytoplasm, resembling foetal gut epithelium. Many multinucleated giant cells were scattered mainly at the periphery of the tumour. Multiple lymphovascular invasions were present. Meta- static nodes revealed carcinosarcomatous feature without heterologous component. On immunohistochemistry, the tumour cells were positive for AFP, glypican-3 and SALL4. Multinucleated giant cells were nega- tive for HCG but positive for CD163. Sarcomatous area were negative for AE1/AE3, positive for vimentin and focally positive for SALL4. Serum AFP was markedly elevated even though it was measured one week after surgery. Conclusion: Although gastric adenocarcinoma with enteroblastic dif- ferentiation is rare, pathologists should be aware of its characteristic histopathological and immunohistochemical findings due to its combi- nation of multiple lineage of differentiation and aggressive behaviour. Our case highlights the importance of careful pathological examination and immunohistochemistry in the diagnosis. E-PS-06-048 Primary alveolar soft part sarcoma of the stomach: a rare case report K. Kim*, J. Ahn *Republic of Korea Background & objectives: Alveolar soft part sarcoma (ASPS) is a rare malignant mesenchymal tumour which commonly involves deep soft tissues of the extremities and trunk. Herein, we report a rare case of ASPS of the stomach, showing preoperative diagnostic difficulty. Methods: The patient was a 32-year-old man who had no past history. During a screening esophagogastroduodenoscopy (EGD), a subepithe- lial lesion was identified in the midbody of the stomach. The following endoscopic ultrasonography and the computed tomography (CT) of the abdomen showed a 2-3cm-sized exophytic mass in the stomach, being suggestive of gastrointestinal stromal tumour. The laparoscopic wedge resection was performed. Results: Macroscopically, a well-defined subepithelial mass with tan-to-yellow cut surface, measuring 2.5cm in size, was identified. Microscopically, the tumour consisted of large polygonal cells with round and vesicular nucleus, prominent nucleolus and abundant eosino- philic cytoplasm. It showed nested growth pattern with fibrous septa and capillary network. These nests revealed central discohesiveness. Immunohistochemically, the tumour was diffusely positive for TFE- 3, focally positive for HMB45, and negative for c-kit, DOG1, SMA, desmin, H-caldesmon, CD34, S100, pancytokeratin, EMA, melan A, chromogranin A and synaptophysin. Based on these findings, it is con- sistent with ASPS. During two years follow-up, EGD and abdominal CT showed no evidence of disease recurrence or metastasis and no remarkable abnormal findings. Conclusion: ASPS is a rare distinctive sarcoma, typically occurring in deep soft tissues. The current case is worthy of its rarity of primary gastric ASPS and added to the literature on ASPS of the stomach. In addition, this case emphasizes the need, even though it is unusual, for clinicians and pathologists to recognize that ASPS could be represented as a subepithelial tumour of the stomach. Long-term follow-up and fur- ther researches should be gathered to evaluate the clinical significance for gastric ASPS. E-PS-06-050 Malignant gastrointestinal neuroectodermal tumour with SMARCA4 mutation L. Korša*, M. Abramović, L. Kovačević, P. Jurešić, I. Veliki Dalić, Z. Marusic *Clinical Department of Pathology and Cytology, University Hospital Centre Zagreb, Croatia Background & objectives: Malignant gastrointestinal neuroectoder- mal tumour (GNET) is a rare malignant tumour which occurs mainly in the gastrointestinal (GI) tract in young and middle-aged adults. This is the first report of SMARC4 molecular alteration in a GNET. Methods: A 54-year-old man with an uneventful previous medical history presented with ascites. Computed tomography scan of the chest and abdo- men showed peritoneal carcinomatosis, thickening of the hepatic flexure and sigmoid colon, liver metastases and abdominal lymphadenopathy. Colonoscopy was not performed due to patient’s poor general condition. Results: Exploratory laparotomy revealed multiple omental nodules, peritoneal carcinomatosis and a large palpable inoperable tumour of the sigmoid colon. The tumour nodules contained solid nests of atypical epithelioid/rhabdoid tumour cells. Mitotic index was 10/mm2. Tumour cells were diffusely positive for Sox10 and MiTF, focally positive for S100, and weakly for HMB45 in few cells. Desmin, epithelial mark- ers and melan A were negative. As the differential diagnosis included metastatic melanoma and GNET, comprehensive genomic profiling using a cancer-related gene panel confirmed an EWSR1-ATF1 fusion and the diagnosis of malignant GNET. Additionally, SMARCA4 defi- ciency with loss of exons 19-30 was discovered. Subsequently, there was loss of SMARCA4 expression by immunohistochemistry. Conclusion: SMARCA4-deficient malignant neoplasms are rare, and typi- cally present as poorly differentiated, aggressive carcinomas involving the lung, ovary, GI tract and rarely other organs. Histology is characterized by dyscohesive, rhabdoid appearing cells with eosinophilic cytoplasm, eccen- tric nuclei and visible nucleoli. Necrosis and mitotic activity are frequently prominent. SMARCA4-deficient malignancies of the GI tract are restricted to rare cases of undifferentiated carcinoma and to our knowledge, this is the first described case of malignant GNET with SMARCA4 deficiency. E-PS-06-051 Collagenous gastritis: a case series and literature review E.N. Kozan*, S. Yüksel, B. Savaş, A. Ensari *Ankara University Medical School, Pathology Department, Turkey Background & objectives: Collagenous gastritis (CG) is a rare disease, less than 300 cases reported so far and characterized by subepithelial deposition of collagen, variable inflammation in the mucosa. We aimed to present five cases of CG and discuss our findings with literature. Methods: Retrospective observational study of all 5 cases diagnosed as CG based on clinical and pathological criteria, in our institution from 2010 to 2023. Clinical information was retrieved from the hospital records. All histological preparations of the cases, including histo- chemical trichrome stained sections, were reevaluated. Results: Four patients in the adult age group were female, and one patient was 9 years old male. Three patients presented with iron defi- ciency anaemia whereas four of them had accompanying diseases including ulcerative colitis, Crohn’s disease, allergic rhinitis, Sjogren’s syndrome. Endoscopically, nodular appearance was observed in the gastric mucosa in 4 patients. Microscopically, CG was found to be accompanied by collagenous colitis in one case while collagenous duodenitis and colitis in another. Collagenous bands were observed in the gastric corpus and antrum mucosa in two patients while only in the gastric corpus mucosa in 3 patients. Clinical improvement was observed from PPI and iron supplementation treatment. Conclusion: Similar to the literature; female predominance was observed and 80% of the patients had at least one autoimmune, inflammatory or rheumatological concomitant disease. Two phenotypes of CG have been