ECP 2023 Abstracts

S365 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 of lymphocytes. Immunohistochemical stains showed positive staining for ck19 in the epithelial cells, with CD 20 and CD 3 positive B and T lymphocytes respectively, the former forming small follicles, mostly representing follicular hyperplasia. The adjacent fatty tissue showed scattered aggregates of residual thymic tissue, including Hassall cor- puscles and lymphocytes with follicular hyperplasia. Conclusion: Lipofibroadenoma of the thymus is a rare tumour with dif- ferent manifestations. Our patient is diagnosed with myasthenia gravis and showed thymic LAF with adjacent follicular hyperplasia. E-PS-23-005 Thymolipoma, a rare mediastinal tumour: pathological study of 6 cases H. Ichrak*, R. Ayadi, B. Emna, L. Farah, S. Hamza, O. Khouildi, A. Ayadi *CHU abderrahmen Mami Tunisia, Tunisia Background & objectives: Thymolipoma is an extremely benign tumour of the anterior mediastinum and accounts for only 2- 9% % of all tumours of the thymus. When symptomatic, they present with non-specific compressive symptoms such as chest pain and dyspnea. Methods: We report a retrospective study of 6 cases of thymolipoma diagnosed at our department of pathology between 2004 and 2022. Results: There were 3 male and 3 female patients, aged between 24 and 48 years, with a mean age of 40. All patients underwent a surgi- cal resection. On gross examination, tumours’ size ranged between 7,5 and 23 cm with a mean of 18,5 cm. They were described as fairly well-circumscribed, soft, yellowish, fatty and lobulated tumours with focal solid areas. Microscopically, all lesions showed similar morphol- ogy: mature adipose tissue admixed with areas containing remnants of thymic tissue and limited by a thin peripheral fibrous capsule. Exami- nation of all sections did not reveal any areas of cellular atypia or mitoses. The clinicaloutcome was satisfactory in all cases. Conclusion: Thymolipomas are rare tumours but should be considered in the differential diagnosis of mediastinal anterior lesions. They pose a significant challenge due to their unclear histogenesis and atypical clinical presentation. Complete surgical resection is recommended in all thymolipomas with favourable survival rates. E-PS-23-006 The 2021 WHO Classification of tumours of the thymic neuroen- docrine neoplasms H. Ichrak*, R. Ayedi, B. Emna, L. Farah, S. Hamza, O. Ismail, A. Ayadi *CHU abderrahmen Mami Tunisia, Tunisia Background & objectives: Thymic neuroendocrine neoplasms are uncommon tumours. They were recently classified by the World Health Organization into four histological entities: low-grade carci- noid tumours, intermediate grade carcinoid tumours, and high-grade neuroendocrine carcinomas including large cell neuroendocrine carci- nomas and small cell carcinomas. Methods: We report a retrospective study of 8 cases of thymic neu- roendocrine neoplasms diagnosed at our department of pathology between 2004 and 2022. Results: There were 4 male and 4 female patients, aged between 32 and 64 years with a mean age of 49. The diagnostic was made on surgical resection (n=5) and transparietal biopsy (n=3). An intraopera- tive frozen section was performed (n=7) showed a malignant tumour process (n=4), non-small cell carcinoma (n=2) and malignancy that may be consistent with non-Hodgkin’s lymphoma or thymic carcinoma (n=1). The histological examination revealed, intermediate grade car- cinoid tumours (n=6) and high-grade neuroendocrine carcinomas (n=2). Immunohistochemically, the tumour cells were positive for synaptophysine (n=8), chromogranine (n=8), NCAM (n=6), EMA (n=3) and pancytokeratin (n=1). However, they were negative for muscle actin, PS100, CD5 and TTF1. Conclusion: The rarity of thymic neuroendocrine neoplasms and the difficulty of pathological diagnosis make it a difficult malignancy to study. Their main differential diagnosis are extension or metastases of their bronchopulmonary counterparts. As there are no known differen- tiating morphological or immunohistochemical features, the distinction relies heavily on clinical and radiological assessment. E-PS-23-007 Lymphohistiocytoid variant of malignant mesothelioma of the pleura: a case report and diagnostic approach I. Provatas*, P. Pantoula, N. Tsibidis, E. Ventouri, A. Karakosta *Pathology Laboratory, General Hospital of Nikaia - Piraeus "Ag. Panteleimon", Greece Background & objectives: Diffuse mesothelioma is a malignant, aggressive mesothelial neoplasm, with a total survival time up to 27 months. The lymphohistiocytic variety is a rare type of mesothelioma and very few cases are described internationally. Methods: A 75-year-old man, with no history of exposure to asbestos or other predisposing factors, comes to our Hospital with breathing difficulties, cough and weight loss. Chest X-ray and CT scan show localized pleural thickening, visceral and parietal pleural nodules bilat- erally, and bilateral pleural effusion, the cytology of which suggested mesothelioma. The patient underwent decortication. Results: All the received tissue portions concerned strongly scle- rotized parts of pleura, within which abundant histiocytic cells were observed, with severe cellular and nuclear atypia, focally with a large eosinophilic nucleus. Binuclear - polynuclear forms and atypical mitoses coexist, on a substrate including many lymphocytes and few plasma cells. Immunohistochemically, Vimentin, CD68 and focal CKAE1/AE3 expression was observed, whereas the lymphocytic substrate expressed CD8, CD3 and focally CD20. Ki-67 index was highly elevated. The differential diagnosis included undifferentiated pleomorphic sarcoma, sarcomatoid carcinoma, melanoma, anaplastic lymphoma, thymoma, and sarcomatoid mesothelioma. The considera- tion of all the above data led to the diagnosis of the lymphohistiocytic type of diffuse mesothelioma. Conclusion: Diffuse Mesothelioma is a malignant neoplasm, closely related to exposure to asbestos as well as to mutations in the BAP-1, MSH-3, BRCA-2 etc. genes. The lymphohistiocytic type of Diffuse Mesothelioma is associated with a dense lymphocytic infiltrate. The overt T-cytotoxic lymphocytic infiltrate seen in this type of mesothe- lioma appears to correlate with an initially good clinical course. There- fore, a regulation of the lymphocytic immune response could be a field of study to improve the treatment of mesothelioma. E-PS-23-008 Mediastinal clear cell sarcoma: a challenging diagnosis of a rare tumour I. Provatas*, P. Pantoula, N. Tsibidis, V. Solomou, M. Karagianni *Pathology Laboratory, General Hospital of Nikaia - Piraeus "Ag. Panteleimon", Greece Background & objectives: Clear cell sarcoma (CCS) is an uncom- mon soft tissue sarcoma with melanocytic features, occurring mostly to young adults, without gender predilection. Most of them arise in the distal extremities and they are very rarely located in the mediastinum. Methods: We report a case of a 52 year-old male with Trisomy 21, operated for a tumour located in the lower left thoracic wall. We received multiple segments of a white-creamed and partially dark brown, solid, elastic tumour, 5,8 cm in maximum diameter,