ECP 2023 Abstracts

S364 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 The diagnosis was supported by positive immunoreactivity for smooth muscle actin and in areas with leiomyosarcomatous appearance, whereas the osteosarcomatous component was negative. A FISH study was performed as well to rule out the main differen- tial diagnosis that is dedifferentiated liposarcoma with heterologous smooth muscle and osteochondroblastic differentiation, and it came back negative (no amplification of MDM2 gene). Conclusion: Dedifferentiated leiomyosarcoma is a very rare disease with poor outcome. It is a diagnosis of exclusion that require knowl- edge, patience, and time. Extensive sampling and FISH study are strongly recommended in diagnosis approach. E-PS-23 | E-Posters Thymic and Mediastinal Pathology E-PS-23-001 Thoracic SMARCA4-deficient undificient tumour A. Demir*, E. Kilic Bagir, D. Gümürdülü *Çukurova University, Pathology Department, Turkey Background & objectives: Thoracic SMARCA4-deficient undifferen- tiated tumour was described in the final who 2021 classification, that exhibits an undifferentiated and rhabdoid phenotype and defiency of SMARCA4, an important member of the BAF chromatin remodelling complex. Methods: We present 61 years old man had been smoking 50 pack/ year, who applied to the hospital with the complaints of back pain and shortness of breath. CT imaging revealed 8x6x5 cm solid irregular left bronchial mass involvement mediastinal involvement areas. In Frozen section, the patient is diagnosed with non-small cell carcinoma and left pneumec- tomy lymph node dissection is performed. Results: 17x15x8 cm pneumectomy material was examined, a centrally located 8x6x5 cm solid lesion was observed, invading the pleura, at a distance of 3 cm from the bronchial surgical margin. Microscopic examination of the mass a poorly differentiated with eosino- philic cytoplasm, vesicular nuclei, prominent nucleoli and plasmocytoid appearance tumour cells and large areas with necrosis was observed. İmmunohistochemically tumour cells were CD34 focal positive, SALL4 focal positive, keratin focal positive and SMARCA4 negative (loss). Conclusion: smarca4-deficient tumours are rare, aggressive tumours with undifferentiated morphology. It should be considered in male patients with a history of smoking at a young age. The differential diag- nosis includes lymphoma, NUT carcinoma, germ cell tumour, neuroen- docrine carcinoma, large cell carcinoma, melanoma, as well as various types of sarcoma such as CIC rearranged sarcoma, malignant rhab- doid tumour, epithelioid sarcoma. Clinical correlation is imperative, as SMARCA4-UTs with a similar phenotype can metastasize to the thorax from external sites (uterus, ovary, stomach, kidney, pancreas). E-PS-23-002 A single-institution experience of primary mediastinal large B-cell lymphoma: a report of 88 cases L. Farah*, R. Ayadi, H. Ichrak, B. Emna, O. Khouildi, A. Ayadi, S. Hamza *CHU Abderrahmen mami Tunisia, Tunisia Background & objectives: Primary mediastinal large B-cell lym- phoma (PMBCL) is a rare large B-cell neoplasm, characterized by a rapidly increasing anterior mediastinal mass, causing compression of the surrounding tissues. The aim of this study is to describe its clinical-pathological and immu- nohistochemical features. Methods: This retrospective study included all patients with a patho- logically confirmed diagnosis of PMBCL treated at our department of pathology between 2004 and 2022. Results: There were 54 female and 34 male patients, aging between 15 and 87 years with a mean of 39,5. The diagnosis was made on sur- gical biopsy (n=55), lymph node biopsy (n=16), transparietal biopsy (n=13), bronchial biopsy (n=3) and surgical resection (n=1). The his- tological examination revealed diffuse sheets of lymphomatous tumour cells intermixed with a moderate inflammatory infiltrate. This prolif- eration is dissociated by a rather extensive and dense fibrosis. Tumour cells are large, with abundant clear cytoplasm, round to oval nuclei, irregular contours and conspicuous nucleoli. Immunohistochemistry revealed diffuse positivity with CD20 in all cases, focal positivity for CD30 (n=15) and CD3 but negative stain with CD15 and CK. Conclusion: The pathological diagnosis of PMBCL remains challeng- ing because of features that may overlap with other types of lymphoma (Hodgkin and non-Hodgkin). However, recognition and diagnosis are crucial for the initiation of optimal treatment. E-PS-23-003 Pseudotumoral mediastinal amyloidosis: a histological surprise! L. Farah*, R. Ayadi, B. Emna, O. Khouildi, A. Ayadi, S. Hamza, H. Ichrak *CHU Abderrahmen mami Tunisia, Tunisia Background & objectives: Amyloidosis is an ubiquitous deposition of extracellular abnormal proteins. An isolated mediastinal location is unusual and typically raises concern for underlying malignancy. The aim is to describe the clinical-pathological findings and discuss the differential diagnosis of this disease. Methods: We report a retrospective study concerning 12 cases of medi- astinal amyloidosis diagnosed at our department of pathology between 2004 and 2022. Results: There were 11 male and 1 female patients, aged between 16 and 78 years, with a mean of 46 years. The diagnosis was made on mediastinoscopic biopsy (n = 8), surgical resection (n= 2) and lymph node biopsy (n= 2). The histological examination revealed extracel- lular deposits of amorphous eosinophilic material often surrounded by foreign-body type of multinucleated giant cells. The material was Congo-red stain positive and showed the characteristic apple-green birefringence when examined by polarizing microscopy. Immunohis- tochemistry stain showed that the amyloid was composed of lambda light chains in most cases. Conclusion: Despite its rare occurrence in the mediastinum, Amylo- dosis should be considered as a differential diagnosis when facing a mediastinal mass. E-PS-23-004 Lipofibroadenoma of the thymus in a patient with myasthenia gravis; a case report E. Hijazi*, M. Al-Hussaini *Islamic Hospital, Jordan Background & objectives: Lipofibriadenoma (LFA) of the thymus is a rare, benign thymic tumour, with different manifestations, with only 6 reported cases so far. We report a case of LFA in a 39 year old man diagnosed with myas- thenia gravis. Methods: Our patient is a 39 year old male, who is diagnosed as myasthenia gravis, and presented with myasthenia crises. Radiology study showed a well-defined, partially- calcified lesion in the anterior mediastinum, measuring 2.3 cm in maximum dimension. The patient underwent thymectomy with complete excision of the mass and surrounding fatty tissue. Results: Histological study revealed a well-circumscribed lesion, showing fibrotic and hyalinized stroma, with narrow strands lined by bland -looking epithelial cells in between, resembling fibroadenoma of the breast, as well as foci of mature adipose tissue and aggregates