ECP 2023 Abstracts

S200 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 E-PS-06-027 Molecular profile of dissected components of mixed neuroendocrine- non-neuroendocrine carcinomas suggest a common precursor lesion V. Fusté Chimisana*, L. Catasús Cols, A. González, C. Fumagalli, A. Prat, D. Piñol, J. Szafranska *Hospital de la Santa Creu i Sant Pau, Spain Background & objectives: Mixed neuroendocrine-non-neuroendocrine carcinomas (MiNENs) are rare and complex tumours that have both neu- roendocrine and non-neuroendocrine components. The origin of MiN- ENs is not entirely understood. We aim to provide evidence that MINENs are a single neoplastic proliferation with different phenotypic features. Methods: Three cases of MiNENs were diagnosed in our institution from July 2021 to August 2022. The diagnosis was made upon mor- phology and immunohistochemical characteristics. NGS sequencing using Oncomine Focus Assay (Thermo Fisher Scientific) or TruSight R Tumour 15 (Illumina) of the two components of MINEN from colon were performed in 3 cases after manual macrodissection. Results: Histopathologically, the neuroendocrine component was large cell poorly differentiated neuroendocrine carcinoma (NEC) in all cases. The non-neuroendocrine component (NNEC) in two cases was moder- ately differentiated adenocarcinoma and mucinous adenocarcinoma in the third case. The molecular profile was different in each patient, but common in both components although harbouring different proportion of allelic frequency, always higher in the NEC component. Case 1 showed mutations in BRAF (NEC 41.8% , NNEC 30.3%); and p53 (NEC 72.4%, NNEC 35.9%). Case 2 harboured KRAS mutations (NEC 58.7% , NEC 30.5%). Case 3 showed mutations in ERBB 2 (NEC 35%; NNEC 13%); BRAF (NEC 34.5%; NNEC 12%); and APC (NEC 27%; NNEC 10%). Conclusion: The pathogenesis of MiNENs is still not clear, and differ- ent theories have been proposed to explain the biphasic morphology of these neoplasms. Our results showing the common molecular profile of the two parts of MiNENs, suggest that both neuroendocrine and non- neuroendocrine components arise from the transformation of a pluripo- tent stem cell that has the ability to differentiate into both phenotypes. E-PS-06-028 Perforation as first manifestation of Enteropathy Associated T-cell Lymphoma (EATL) L. Gallego*, F. MacSweeney *University Hospital Waterford, Ireland Background & objectives: EATL occurs in middle age patients with Coeliac Disease and represents less than 1% of Non-Hodgkin lympho- mas. An early diagnosis and treatment of Coeliac Disease can decrease the risk of lymphoma. Methods: The case is one of a 73 year old male with no previous enteropathy diagnosis, who underwent a bowel resection due to small bowel perforation at UHW in August 2022. Further histopathology analysis produced a diagnosis. Results: Grossing of the specimen showed perforation associated with a fibrinous exudate, adhesions and mesenteric nodules. On microscopy, a large lymphomatous tumour mass arising from the small bowel involv- ing mesentery and appendix was found. Loss of villous architecture and increased intra epithelial lymphocytes were noted in the background. The lymphoma was composed of medium to large cells with atypi- cal features. Sampled lymph nodes were reactive but with no obvious involvement. Immunohistochemistry was positive for LCA, CD3, CD8 and CD30 (focal), with a Ki-67 index of 70%. Subsequent bone marrow aspirate and bone core biopsy were inconclusive and could not exclude the possibility of low volume involvement by T cell lymphoma. Conclusion: EATL is a very rare type of Non-Hodgkin Lymphoma, which is associated to Type 2 Refractory Coeliac Disease. Staging is as per in other lymphomas, but as it is usually found as an advanced disease in laparoscopy, treatment is challenging and can combine surgery, chemotherapy, stem cell transplantation or biologic therapies. It is important to keep EATL between the differential diagnosis of GI lymphoma, and assessment of the background mucosa can provide help- ful information. E-PS-06-029 Use of deeper sections in the assessment of complete regression in rectal resection specimens J. Gama*, S. Pessoa, I. Rodrigues, B. Sepodes, C. Courelas, A. Alves, J. Pimentel, F. Ramalhosa, J.L. Amaral, J. Madeira, V. Almeida, M.A. Cipriano *Centro Hospitalar e Universitário, Portugal Background & objectives: Neoadjuvant chemoradiation therapy in rectal cancer is associated with downstaging. To report tumour response to chemo- radiation amodified Ryan scheme is commonly used. In this study, we sought to find whether residual tumour could be found resorting to deeper sections. Methods: All rectal resection specimens at our institution (2015-2021) were reviewed. From these, the specimens submitted to chemoradia- tion therapy with complete response (Modified Ryan Scheme for tumour regression - score 0) were selected. In these cases, deeper cuts at a depth of up 30μm were requested. Results: From a total of 425 rectal resection specimens submitted to chem- oradiation therapy, 50 specimens with complete response were selected. A mean of 6.98 sections per case were studied (median:5 sections, range 2-40). Even after deeper cuts no evidence of residual tumour was found in none of the specimens. Conclusion: In our study, the use of deeper sections in the evaluation of rectal resection specimens did not change the staging after neoadju- vant therapy, therefore showing that the tissue observed in the first cuts was indeed representative. In departments with extensive sampling of the tumour bed, one section of each block is representative. E-PS-06-030 MUC5AC and PDX-1 are useful markers to identify colonic polyps of the serrated pathway M. Gené Hijós*, E. Fernández, M. García, S. Roca, M.S. Martinez, E. Musulen *Pathology Department, Hospital Universitari Joan XXIII, Tarragona, Spain Background & objectives: Gastric metaplasia (MG) was described as the first event in the adenoma-carcinoma progression in the serrated pathway of colorectal cancer. The usefulness of MUC5AC and PDX1, markers of gastric epithelium, was investigated to properly classify conventional and serrated colorectal polyps. Methods: A series of 229 colonic polyps were collected and character- ized as follows: 74 tubular adenomas (TA), 72 serrated sessile lesions (SSL), 70 serrated sessile lesions with dysplasia (SSLD) and 13 tradi- tional serrated adenomas (TSA). Serrated histological features and type of dysplasia were reported. MUC5AC and PDX-1 immunohistochemis- try was performed in all cases and the results were statistically analysed. Results: MUC5AC was positive in 116 (75%) of serrated polyps (66 SSL, 41 SSLD and 9 TSA) and in 12 (16%) TA (p=0.000). PDX1 was expressed in 154 (67%) of serrated polyps (64 SSL, 58 SSLD and 12 TSA) and in 38 (51%) TA (p=0.000). When we evaluated the relationship of both MUC5AC and PDX-1 expression as a function of serrated characteristics, such as the presence of serration, dilated crypts and presence of mucin, the results had statistical significance for both markers (p=0.000). In addition, the different expression of MUC5AC and PDX-1 to differentiate TA from SSLD showed high statistical sig- nificance for the two markers (p=0.000). Conclusion: MUC5AC and PDX-1 expression is associated with ser- rated precursor lesions. Both markers are useful to differentiate serrated polyps from conventional when characteristic features are difficult to assess due to poor specimen orientation or fragmented resection of the lesion, even when dysplasia is present. Our results corroborated